Sorry to hear this, Kerb. There's not much you can do about cutting back the ivy if it is on the other side of the fence and, really, it is not suitable for a wooden fence anyway, because it is very heavy. The real answer is to get it out by the roots and that's not easy, either. A solicitor's letter? We are not on chatting terms with our neighbour, either, so I sympathise, but a polite request and an offer to pay for someone to take it out might work. I imagine that if his ivy damages your fence, you have a claim on him to repair the fence, but a solicitor could advise you there.