Water heater tripping out

I was prepping the van today ready for our weekend away, and for some reason the water heater keeps tripping out.  It's a Truma water heater in a Bailey Orion.  I've ensured it's full of water, I've checked the fuse in the fuse box, and have flicked the trip switch on and off.  The heater ignites but click off after 10 seconds or so, and a red light illuminates next to the heat selector switch (the one where you choose 50 degrees or 70).  

We had a motor mover fitted a few weeks ago, and a couple of weeks prior to that we had the blown air heater replaced.  Both these items are physically nowhere near the water heater, but is it possible that the engineer has somehow 'broken' the circuit to the water heater??  I don't want to make any false accusations.  Otherwise, the fridge is on, lights are working so the rest of the electrics seem ok (except for 3 point plug sockets aren't producing any power but I guess this is normal if the van isn't plugged into the car or EHU)

Any advice gratefully received.