I'd suggest drafting some sort of response to the solicitor's letter along the lines of:
Dear solicitor,
It is good to see that my ex now wishes to be a part of his daughter's lives. As he has had no contact from (whenever he last made any sort of contact) up until the present time and he chose to move 350 miles away, I am sure he will agree that the first step in the best interest of his daughters would be for him to commit to re-building his relationship with them.
To that end, perhaps he could phone / skype them for (x amount of time) every Friday at 7pm. If he were to write to them and / or send photos and / or cards, I shall do all I can to have the girls respond.
Should this go well, then perhaps after six weeks direct contact could begin in the form of Mr Twat-Ex coming down to (place where we live) and seeing his daughters on the Saturday from 10am until 6pm the next day on a fortnightly basis, on the proviso that will have somewhere suitable for them to sleep over. Otherwise, he could see them from 10am till 6pm both days. (Or whatever length of time you think is appropriate for your daughter's ages)
I am aware that their right to be maintained by him his separate to their right to have him in their lives, but I hope that he can now also show a commitment to providing an appropriate amount of his income to their upkeep.
Yours sincerely,
PWC who won't be intimidated by a solicitor.
And look into McKenzie Friends, too, even though there are the occasional ones with an axe to grind. They are volunteer lay people who will give you support during the court process, including coming into court with you. Usually they've been through it themselves.