So how did you get on with your free hour?
Would point out that your ex may have stopped paying maintenance as he was sacked for gross misconduct... Therefore if he had no income to pay maintenance from he wouldn't have to pay any...
I think you can ask now if he's found himself another job and if not when maintenance will resume. Where you going through the CSA before?
It might be that he doesn't have a job still and is making the most of it and claiming legal aid, hence why the letter from his solicitor. His new partner may be supporting him too. But he will presuumably be claiming IS if he is unemployed so you should still get £5 a week from him if he is.
I would suggest too that the letter has been prompted by his new partner. She is probably keen to help him have contact and he's probably painted a not too good picture of you. If you ignore the letter you play into his hands re his probable stories of you being obstructive to contact and difficult.
If you write a nice reply with offer of resuming contact and appear reaonable throughout she'll start to wonder why he said those things about you...
A letter along the lines of Pinkcars suggestion would be an excellent response. Think hard about what contact is reasonable and can be expected given the distance and dd's ages. Be prepared to offer non direct contact given the distance it makes sense to agree skype time a couple or more times a week. Ask him to record some story tapes he's read onto CD so they can listen to them at bed time. And to write to them and suggest items he could include for them such as stickers, hair bobbles and the like.
Also think about what direct contact would work for you all. For example instead of a full week or two with him which you don't feel the dd's could cope with yet, how about suggesting that once he has re-established contact and been to visit them and taken them out for the day a few times he could then stay in a hotel/b&b to make a weekend of it for them and see them over 2/3 days. After that he could consider a long weekend break somewhere nearby where they stay overnight with him. Then, if that goes well and moving forward, he could take them to his place for a weekend every 2 or 3 weeks if they cope with the journey. After that then you will look at longer times in the school holidays. But as a first off then no.
He's probably told his new partner and the sol he's spent loads of time with them in the past and forgotten the big gaps in between or thought about how all this will affect the dds.
Point out that you consider this to be in the dds best interests and that you are thinking of their welfare in your letter so it is clear that you are considering the dds and that it's not about you or your feelings on this.
Re the new partner's ds - I think you are worrying on some hear say at this stage. Until you have some concrete grounds for concern then you just have to accept that while dd's are with your ex he may well introsuce them to friends of his, including his new partner and her ds. The dc may get on fine, they may not. But you can't speculate on that or have pre-conceived ideas at this stage. His new partner's ds being a handful shouldn't be a reason to refuse contact with their dad. Unless there are wellfare issues a court wouldn't consider this a good reason, especially as you're only basing it on hearsay anyhow.