Reply from Ray Hooley this morning:
"1HR 288869, rated 5.5hp/550rpm, was sold on 19.1.1950, to Watford Technical College, Little Cassiobury, Hempstead Road, Watford.
I don't have the erection dates, but it would have been erected towards the end of 1949."
So a 1940's engine by a few days
It is Watford UTC these days, but I have contacted them to ask about any archive material they may have for the engine. Ray has also sent me a package of info.
Peter