I finally installed an Eberspacher Diesel Heater D1L its a basic model that has an on off Thermostat switch. I took it to a firm in Leicester 'A.B.Butt' for testing. It works great and heats up very quickly. The only drawback is, it will only start with a fully charged battery. But once started it will carry on for quite a while until the battery is down to a 60% charge. The trick (I've been told) is to start this heater when the engine is running, then turn the engine off. I have one 110amp/h battery so will test the heater more fully when I'm on the water.