Sorry to hear that, Edd. If you have got legal cover on your house insurance, try them first.
Other avenues you could try are:
Citizen's Advice
Free half hour consultation at a local solicitors
Some form of mediation
moneysavingexpert.com forum (brilliant forum, they might be able to offer further places to try too)
best to keep any details as impersonal as possible, whichever forum you post on
Not sure about ChatGPT - a good starting point but don't know about the accuracy.
At the end of the day you WILL need to see a Solicitor if you think it can't be resolved amicably but doing research and gathering as much information beforehand might save some money and also give you an idea of questions to ask
Good luck