Carver heater, keeps tripping out

This is one of the two faults with our '98 Bailey ranger im trying to get to the bottom of, this being the most anoying as we're having to use gas for heat even when there is electric on site that's already paid for!

We have a carver fanmaster in the 'van, the gas works fine as it should, and the fan works fine as it should. However when we try and use electric heat, it works for a short while before tripping it's self out. At this point we have to take out the draw to get to the back of the unit and hit re-set manually on the unit, at this point it works again for a short while, before tripping out again. 

Im doing some searching on google but coming up with nothing. All i can find is that Carver dont make caravan heaters anymore! :(

Anyone have any ideas what might be causing it and if/what the repair might be?

Many thanks

D