Ok,
I changed the housing and refitted everything, then found a leak where the bottom radiator hose connected to the thermostat housing elbow, got a new pipe today and fitted it, no leak now. Filled and bled the system and ran it up to temperature in the garage, it got pretty hot (indicated 100+ before the fan kicked
When it did it cooled down quickly, so I took it for a spin on to the motorway, the gauge sat at 110 the whole way (about 5 miles) I stopped before doing the return leg, still 110 indicated, but checked the engine bay no cooling fan running
top pipe to radiator was hot, bottom & the radiator itself cold, thermostat is new.
I did the return run and the temperature stayed pretty much constant at 100 to 110, thought it was ok if a bit hot and confused about the fan, but when I got home it had lost some of its coolant still no radiator fan, let it idle and the fan came on
I am wondering if perhaps the waterpump is not circulating the coolant which is why the engine is hot and the radiator is cold? At idle gravity may be enough to make it circulate which could be why the fan comes on at idle but not on the road?