Thank you all, of course it's a blanking cap: Think I may have just had my first senior moment
Had been fettling the OH Fiat 500, checking levels etc, which in common with most modern cars doesn't have an accessible rad, just an expansion tank. I think being presented with both expansion tank and radiator cap in the Morgan engine bay, coupled with the first real sunshine here this year had me cooked for a bit! Yes, blanking cap on the rad, expansion tank to the side, with 15lbs pressure cap; all correct and present, and run up to pressure and temp after fixing the subtle pfft ppfft of a pinhole in the overflow pipe from the rad filler neck to the expansion tank! Thanks for helping clear the confusion: I'm off for a lie down....
