Dear Neil,
If your neighbour was the one who retained/contracted the surveyor to do the work then he is liable to pay the surveyor. I can't imagine what basis he would have to claim a portion of the fee from you, unless you had come to a prior arrangement regarding this. I would write a firm response to the letter of demand stating this and making it clear that it was his decision to have the survey undertaken, no prior notice or agreement was sought from you to contribute and therefore he is liable to pay the fees himself.