Thank you for the replies. So speedy at this time of night!
TMOAD I will have been with my DP for 4 years next month. Despite not living together full time, we have been together alternate weekends and most of the school holidays during that time, either at my house or his. The intention has always been to move in together and would have happened about 2 years ago but his work field is very specialised so he can only work where he is, and I was at uni doing quite a specialised degree which I couldn't transfer as only a handful of places offer it and none near him. Backing out of living together won't be terribly simple either, since I am expecting his child 
so it is certainly something that will be worked at.
CJdamoo that sounds pretty similar to my circumstances then. Does your DH having PR give him much the same rights as adoption would? I am having to move schools for the DC since we are moving so far and he can't fill the forms in to get things started as he currently has no rights or responsibilities at all. That is the sort of thing we want to enable him to do. PR and deed poll are our back up plan if adoption isn't granted or would be too much trauma for the DCs.
Prh47bridge that is interesting to know. May I ask your source about needing to be married for PR? It isn't that I doubt you at all, it is just completely different to what I understood from the directgov site so if I have misunderstood that, I need to double check my understanding of everything else so it would be great to have another source to go to!
I am aware that 'we want to be togevver' doesn't qualify as a valid reason for the courts to approve an adoption, especially when the birth father has (albeit limited and infrequent) contact with the DCs. However I do believe that we have a good case for applying for adoption since (and again, this may just be my misunderstanding) that seems to not only grant my DP rights but also to remove them from my ex.
Without wanting to bore you with details, my ex was abusive to me in front of the DCs. DS2 has only just, after 7 years, overcome his phobia of having things touching his throat after seeing me be pinned against the wall, off the floor, by my throat when he was 2. He also stole from them to support his drug habit, has never had sole care of either of them, even for an afternoon when we lived together, because of his drink/drugs problem. He had Class As in the house when the children were there (not the fun party ones either). He absconded with DS2 one day after we had had an arguement over the phone, he drove into town, found me and snatched DS2 out of his buggy and drove off. He then hid when the police arrived at our house. Obviously this is all on record and is the main reason for XP only having supervised access in public places. XP has been threatening and abusive to me on a number of occasions since we left and quite regularly threatens to take me to court for access/residency/etc and makes these threats to the DCs to pass on to me, so they are aware of it all.
So, with all that in mind, I have been to the solicitors a number of times over the years and each time been told that he would never get so much as an access order, since the courts would see that I offer more than they would give and he doesn't keep up with even the minimum they could require of me and so not waste their time taking it to court. However, that hasn't stopped him trying and involving the DS's in his attempts. That is why I am keen to revoke his potential claim to future rights, so that he cannot continue to use them and upset the DCs. I am not proposing stopping contact, so long as the DC want to which has always been my condition. Another change recently is that they simply don't want to. I think after seeing with DP what a real Dad is like, they have realised that their own was sadly lacking and that they don't have an obligation to him to visit. It may just be that they are at a selfish stage as well though so obviously it is something we would continue to offer and talk over with them over time.
Sorry, that was really long again. I was trying to avoid putting too much detail in my initial post and boring people but seem to have done it here instead 
KristinaM I think I may have to give up on the directgov site, as I gleaned from there that it is only 6 months before we can apply for PR, unless the 2 years is how long it takes for the entire process to reach completion?
I do agree with you all that there is a lot going on with us right now, new house, new school, new living arrangements and new sibling (well, 2 but one who they actually live with and see) and ordinarily I would agree that it would be best to just let it be for a while, however I think I am feeling a sense of urgency as my FIL (DP's Dad) recently died very unexpectedly and it has brought home to me how fragile and short life can be and also that if I were to die tomorrow, that there would be no one with a legal claim to my DC and even had I been living with DP for 3 years (for example) and they were settled with him and their new sibling, that if I die there is nothing stopping their birth father applying for custody of them. I am quite certain that it would be a cold day in Hell before they were given to him over the man who has been a father to them, and taken out of their home and area to be sent to a man who they barely know who has 4 other children to care for. However, given how fresh the memories are of helping to sort out my FIL's estate, I just don't want to add that level of stress and uncertainty to it. My FIL was a highly organised man and we could lay hands on every single piece of paper we would need within minutes of wanting them and it was a straightforward case, but it was still a lot to do and I don't want to lumber my DP with that, the sole care of 3 mother-less children & a potential court case as well.
I realise that is incredibly morbid though 
Again, thank you for your replies, especially at this late hour, and apologies for rambling. Is this the right place for this thread btw? I have since found the Adoption sub-forum and don't know if there would be more appropriate?