It seems I may have done the guy selling the Hobbs on Ebay, Paul Cameron, a dis-service in doubting its authenticity. I got a response from him effectively saying that the bike is one that was built by Youngs of Lewisham for Hobbs and thus didn't conform to the traditional Hobbs frame numbering sequence. In other words, it isn't an in-house Hobbs.
As people who contacted me off-list about this bike now know, I had good reason to doubt its authenticity although it appears, in this case, that Mr Cameron is selling a genuine machine.
'Nuff said.
Bruce
Dundee
Scotland