Hello Stephen, Your difficulty sounds similar to a problem that afflicted Carver Cascade Water heaters. Over winter there can be some water left in the cold water inlet pipe (being one of the lowest points in the caravan). If the water freezes, it can displace the O ring seal on the inlet non-return valve to the heater. This stops the valve shuttle moving and prevents water from entering the heater. To check, go to the outside cover of the heater and release the drain port. Allow any water in the tank to drain away, then turn on the pump. You should get a strong gush of water from the drain port. If you only get a dribble then it is likely the Non-return valve is faulty. Replacement of the valve or the O-ring can be DIY, but if it involves breaking gas joints then I strongly suggest you have a competent fitter remake the joints to keep your insurers happy.