Accounts are apparently overdue... But it does rely on the relevant inspector in the IR picking up on something suspicious. Cash sales are perfectly normal in business.

Also, companies with turnover below a certain limit no longer need their accounts audited - so they could file any old rubbish, as long as no-one investigates. Same as self-assessment income tax, they take your word for it, but investigate a sample (Randomly, they SAY).

HM Customs & Excise are another matter, but as long as you declare what looks like the correct VAT on sales, they are happy.

Surely the MU could take action on behalf of one member, even if it is against another member?