Thank you guys!
We have never had enough hot water since we bought the boat, it is only recently that I noticed the temp guage showing her running cool(ish) but that is quite possibly how it has always been.
The engine is a Nanni 4220 HE, as far as I know it is the boats original - 1985 Broom 29
The calorifier is horizontal but as near as the eye can tell the higher hose is level with the engine, photo attached where you can see the hoses running parallel and level.
In my first photo the return hose is the one marked 2, it goes into the base of the housing you can see where the brass is showing, as far as I know this houses the impeller ??
Am home now but can get more photos tomorrow pm, I haven't got a user manual though I'm afraid.