He has so far identified two such questions:
#1: Who is paying school fees in cash?
The advantage of asking schools for such information is that the identity of the payment has to be unambiguously associated with a named person and address.
A commentator on Richard's blog suggests that it might be equally worthwhile to ask travel agents who has paid them in cash.
#2: Who is paying private medical fees in cash?
Again Richard notes that the advantage of asking private hospitals is that the identity of the payment has to be unambiguously associated with a named person and address. Such disclosures would not breach medical confidentiality – the detail of the procedure is not the subject of the enquiry.
I tend to agree that such information would provide useful ammunition for HMRC in the fight against tax evasion. Of course the private schools and private hospitals would be constrained from providing the information unless there was a legal obligation to supply it.
This may well be the case using the powers given to HMRC by Schedule 36 FA 2008.