With grateful thanks to Brian for the news about the 165 on the entrance posts of 42 Clarendon Square
A minor revelation was revealed to me that the former Low house at 42 Clarendon Square had been used by the BBC for exterior shots in the latest "Upstairs Downstairs" period drama.
The house is supposed to be 165 Eaton Square in London but I can only assume that it was less expensive to use Clarendon Square in leamington Spa.