@Moray @fesshole Sure, there's nothing wrong with having £90,000 in cash per se, but the cash was obtained through illegal activities (smuggling tobacco), which may count as money laundering.
Assuming the grandfather is dead, then this money could count as inheritance/part of the estate, so the fesser (the heir) can't be held liable for the original illegal activities, but they might still be under scrutiny regarding the legitimacy of the funds. But yeah, the fesser should 100% talk to a lawyer about this, don't listen to my (possibly wrong or outdated) ramblings.
@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.