Its District Court so at least the costs that a Court can award are relatively low. I think you know the answer - you need legal advice and that will cost you. It may be a try on but if, as you say, your vendor's solicitor has a reputation for pursuing cases..........Anyway, they can only do so on the client's instructions.
A Court may well find that , even if the money is technically and contractually due, that given the length of time that has elapsed that it will not make an Order. Or it may make an Order.
mf