Just had a similar situation…
I own a flat. Freeholder of the building dug out the foundations to put a new property in the basement. My flat has cracks in every lounge wall. Here’s what I learnt:
1. I spent £300 getting a structural survey done which confirmed the cracks were a result of the works. Worth doing for piece of mind.
2. I spent £500 on legal letters that he ignored. However he has since said he will “put right” all works. There is no timeframe for him to do this. Waste of time.
3. He should have issued a party wall notice before commencing the works. this would have given us chance to appeal. He didn’t. This is a legal requirement although council have since told me more and more people don’t bother. Read up on party wall notice.
4. I could have taken him to court to get his works stopped as he’d failed to issue this notice but it would have cost £3k approx to take it to court. I had the money but it was allocated for my wedding so I didn’t because…
5. You only get monies refunded if they are found liable and they go over £5k.
Current situation is we still have cracks. He said he will fix them but has no timeframe to do it. The report I had done suggests plaster needs knocking off but unless I go legal he can just fill the cracks. Whole things a mess really.
I contacted insurers but once they knew the report said no serious structural issues, just slippage they weren’t interested.