At the end of the day they are your trees. If you keep them in a condition that makes them unstable, knowingly, then it's down to you. The tree surgeon would make a great witness for the developer if the trees were cut back to your boundary and then fell over (whether they would or wouldn't is another matter and an unknown on the details provided) and then you tried to sue him.
To establish where they may be be cut back to, under common law, you'll need to establish ownership of the footpath. Saying that the footpath has been given over to the council could be that he still owns the land and there's a right of way (maintained by the council) on it. If this is the case he has a common law right to cut back to the boundary between your property and the footpath.
If he has 'gifted' the land and it's owned by the council, then the 'right' is the councils, not his.
Generally, the neighbouring landowner can't force you to cut your trees back, they can exercise their rights and do it themselves, unless the nuisance is 'actionable' - the encroachment is causing actual, physical damage; and in that situation they actually need a County Court injunction of abatement.Edit: The local authority/highways authority operate under different legislation where there is a statutory duty to maintain passage, so they can tell you what to do or do it themselves, a different circumstance altogether.
Others might (will) disagree, but my take on it is that the neighbouring landowner doesn't or shouldn't have to suffer the nuisance of encroaching limbs and roots, whether ordinary or actionable. They have a common law right (in the absence of statutory protection) to self help/abate it. If this then leads to loss of the trees, which you have (admittedly) allow to 'trespass', who is at fault?
I'll borrow Daltontrees disclaimer, if I may?
I post on Arbtalk for free mainly to help further my and others' understanding of trees, without prejudice to any of my professional work. Quote and rely on my postings at your own risk