The swap was 'blessed' by the District Valuer (part of the Valuation Office and an executive arm of HM Customs & Revenue) who proposed that the garage and the barn were of equivalent value and, as such, could simply be exchanged, one for the other. Thankfully I have never had to use the DV's services and, based on that example, I'd avoid them at all costs.
Some of the documents relating to the exchange are reproduced below and they make interesting reading, especially the bit where Mr Keenan is threatening to sell his garage to someone else. Obviously the threat was enough to get SSDC quaking in their boots.
Till next exchange, I'm still Niall Connolly