Thank you for all of your replies. So much advice, so much wisdom, and so many people taking time to think about my situation and reply. I am really, really grateful and you have given me a lot to think about.
champagneandfromage (wonderful user name!) - No, that is not the relationship I want. What I said in my earlier email - the one which they found so offensive - is that a relationship based on 'financial support' is not a relationship at all, but that I have 'an abundance of love' to give. Gut-wrenching as it is, the deeply unpleasant response makes it clear that the love appears to be of little value, and that the only useful thing we have done for them is provide a sum of money, with the prospect of more to come.
SirVix - this is my great dilemma and the reason I want to have the meeting in March, although I suspect it may be painful. I need to see for myself how things are. Is DD instigating this course of conduct, is she complicit, or is she acting under LB's influence? My own view is that she is complicit, but that matters would not have gone so far without LB. I genuinely believe that she mourns the loss of the relationship with her family. I know her to be manipulative and sometimes careless of people's feelings, but I do not believe her to be deliberately cruel. I have not spoken to her on the telephone for a while, and I don't really feel up to it right now.
Random - I agree that it would be better to pay a monthly amount and perhaps you are right to have the discussions before. However, I would like to meet my DGDs, even if it is only the once, although I know, at some level, this may not be a good idea. If we are to walk away, or be sent away, it is better to do so before I become emotionally attached.
Sir Vix - I agree that they may ask for a sum of money and that, like any blackmailer, LB will be out for more. However, one thing I am determined about is that all conversations concerning finances should go through DH. He will be rational, sensible and consistent, and I have total confidence in him to handle it as well as it can be handled. He also has clarity of vision. He says that we should pay the rent, but that is the extent of any contribution. As you say, if DD genuinely wants to do the course, she will do it anyway, and I do think that it would be good for her. If we help in that way, we will have done as much as I believe we should be expected to do, and possibly more than many parents would do. I entirely agree that LB hates me. He wants to make the whole family, but especially me, suffer as much as possible, but he wants to extract money first - the money he thinks he is due from my having pulled out of the previous property transaction. I don't have any illusions in that regard.
billy, ddraig and springy - thank you for your supportive messages.
Random - I have thought of registering an interest over the property which they bought partly with the 'loan' from us, but it would be deeply inflammatory and DH doesn't want to. The reality is that we do not expect that money to be repaid.
Sir Vix - as ever, you are extraordinarily accurate in your conclusions. DD1 actually said to DH that he hadn't done anything wrong, but that he hadn't done anything right. LB does hate us and nothing we do or say will change that. He will tolerate us whilst he thinks we may be financially useful. That is why I want DD to do the course, so that she might gain a different perspective, from meeting other people and being exposed to different points of view. I agree, to an extent, that the realisation that I can't change anything is liberating, once I have come to full acceptance. I am nearly there, but I have relapses.
Clover - thank you for your post. I do feel that I am in an abusive relationship, albeit indirectly. DD2 says, probably correctly, that LB, or both of them, have identified me as the 'weak link'. I used to act for women in DV relationships when I was a newly qualified barrister and, full of the confidence of youth, I could never understand why women would not only become embroiled in these relationships, but keep going back for more. Now, I do understand, all too well. And I never thought that I would be that person. I believed that I was strong - I guess I still need to believe that and to dig deep to gather up those inner resources - to stay strong and calm, as you advise.
mooberry - thank you for your support. Yes, it is disappointing not to be permitted to see DGD2 until she is nearly three months old (when DGD1, whom we still haven't met, will be over 20 months old). A large part of me does feel like walking away, to be honest, but the other part of me wants to go on the visit, kill or cure. I need closure, one way or the other. I have a busy February, lots happening, so am determined to focus on that and it's good that there are lots of things happening to keep my mind occupied.
SirVix - thank you for your further message and for the support.
Gutterton - thank you for your advice. To be fair, most of the money for the two properties has come from him. His grandparents gave him enough to buy the first house, which they renovated and sold at a profit. The second house was purchased mainly from his bursary - our contribution/'loan' was about 25%. I think that is what enrages him, makes him feel bitter and 'sold short'. He believes we are 'rich' and heartless for not helping a young family who need the cash, a narrative which DD1 has bought into. He paints his family as good, who care and who help, and ours as bad. We are being presented with this wonderful opportunity to have access to 'their' family and it is up to us to prove ourselves - at least that's how he sees it and probably DD too. A previous poster said that we were being set up to fail, and I believe that to be true. We could hand over our life savings and still be found wanting.
Grohn - thanks for your support. I love DD2 and DD3 with all my heart and I am determined that they will not lose out because of the problems with DD1, either financially or emotionally. Their love and protectiveness has shone through and I know that I am truly blessed. As they say, two out of three is a good result 
Puzzling - you make some perceptive comments and ask some very pertinent questions. With regard to the financial contribution, DD1 said, just over a year ago, that they were 'scrimping and saving' and why wouldn't we just give them 'a leg up'. She was talking about doing the PGCE and I said that we would support her in that, as I thought it would help her to have a profession, where she could become financially independent. I didn't know about DGD1 then or that they were in a position to buy a second property. I assumed that LB would carry on from his PGCE to become a teacher and that we would give some help to the couple to progress. It's what many parents do and I still felt bad at pulling out of the original property transaction. DH feels - and I agree - that we shouldn't renege on this, or it will be more evidence that we are not to be trusted. DD needs to prove to LB that we are not the terrible parents which he has painted us to be - unlike LB's family, we have not welcomed him into the family and we make promises which we do not keep. Trust me, that is the narrative, and DD believes it. She was happy that we gave them the 'loan' for the property and agreed to help her with her PGCE, as we had proved ourselves enough for us to be worthy of contact and even being introduced to our DGDs. She would like a normal happy relationship, I am sure, but we need to 'earn' it, in LB's view, so she wants us to capitulate so that she is free to engage with us.
My relationship with my parents was very good, close and loving, although their relationship with each other was not. Also, my brother had a number of issues and caused a lot of heartbreak. There was an estrangement as, although my mother loved him deeply, she couldn't cope with him any more (he was 25 when he left). DM also had a lot of serious health issues. She wanted to know that he was alright but that was all, as she had reached the limits of her endurance. I saw her pain but I didn't understand it - until now. She died following a house fire (she had emphysema and inhaled smoke) and my DB took his own life 18 months later. Sorry if this is too much information and I'm not sure how relevant it is, but you asked about my relationship with my DP. I would say that they loved me, were very proud of me, and were more dependent on me than I was on them. But I loved them very much, my mother and I would speak every day, we had a deep bond, and I sometimes feel that I am turning into her.
I have lots of friends and interests, a very full life. I have just completed an MA and have signed up for a Creative Writing course. The next few months are pretty full, seeing friend, theatre, travelling etc. I get what you are saying though, that I need to let go, that they are adults and need to live their lives, for better or for worse. I know you are right and I am getting there, slowly but surely. I would go to a therapist if I thought I could find one who would help me. DH thinks it's a bad idea but DDs 2 and 3 think I should and that DH doesn't really understand mental health. Please feel free to PM me - that would be very kind.
Random - as ever, you make excellent points. A previous poster suggested just paying an equal sum of money to each DD and maybe there is some merit in that idea. That way there would be transparency and fairness and it's up to them how they spend it. If LB invested it in property, at least DD1 would be entitled to a share, as they are married. But then I suspect she wouldn't do the course and LB won't get a job and she will be even more trapped. I don't know.