Hello Bob, is that the one that was on eBay recently? It looked a bit distressed, but worth saving.
You should have a look on the chassis, probably on the left hand chassis rails somewhere not far behind the cab, for the Ministry of Supply chassis plate. If it's still there it'll have the service registration number, contract it was supplied under, and so on, which will be a starting place to get the service history and date into service.
There are a lot of keen bus enthusiasts around, so if you can find a relevant group or forum I'm sure someone will be able to give you its bus company history and perhaps even provide a link back to the military history.
The chassis number is 32616, which I think would make it early '60s, and the model type is RLC3, which decodes as R type, Long wheelbase, (supplied as) Chassis & cab, 300 Petrol engine. Unfortunately it may be difficult to get the exact date of manufacture without the military history or registration, unless someone has a reference to that chassis number. Vauxhall Heritage have chassis number lists but they're not tied to dates.
Great trucks, R types, I'm sure you'll have many hours of fun & pleasure with it!!