My grandad was quite a successful tobacco smuggler when he worked on the coaches, never got caught, and made a fortune. Which he hid. And we found when we knocked down the extension. Cavity wall was stuffed with about £90,000 of old £20 notes. Now we to work out how to exchange.
@fesshole I'm not a solicitor, but if you inherit the proper do you not also inherit the cash hidden in the walls?
I'd be tempted to go back to the solicitor who handled the estate in the first place and tell them you've found some cash (but not how much, if you can avoid it, and definitely don't mention how he acquired it).
You might lose some of it to CGT but that's probably worth it in order to make the remainder usable.