Hans seems to have fallen asleep on the 'return' key!
I'm totally confused as to why HMRC should be involved with the building society accounts. Building societies don't just 'close down'. They get converted to banks, merged with other societies or otherwise taken over in some way. There's always a 'successor' and it's that successor who the solicitor should have contacted. For example, if one of the accounts was with the Woolwich Building Society, which no longer exists, then it's Barclays Bank that's the successor to it. All the solicitor needed to do was a quick google search to find the names of the relevant successors and then to approach them to release the funds (complete with any relevant interest).