Update; took the bike back to the dealer who had done the 16k service just a few days ago. Rad fine, hoses fine, pressure fine, no leaks so false alarm. Seems as though when the rad get wet then the bike is run up to operating temp and the fans cut in (98 deg C) the fans pull the water off the rad fins and squirts it onto the exhaust. Hence the steam. Coolant down by 50ml so there may have been a small air bubble too.
So big up to Lings of Harleston and Ian G, the tech. Didn’t charge me either.