Apparently, according to
this thread Bristol recommend a 15lbs radiator cap.
I would question why it needs to be that high, given several reports of popping core plugs on these engines, but maybe it has something to do with the design of the waterways in the engine.
Of course you need to make sure the valve in your rad cap is actually opening at the correct pressure (not sure how you test that!)
As for the presence of an expansion tank, another 408 owner will need to confirm that. (There is one fitted to the 409 but the 408 Mk2 parts list does not show one. I don't have a 408 parts list).
There is one on the attached pic of a 408 engine bay, but it's doesn't look original.