Truma Ultrastore repair on Ravenna.

My Ultrastore won't ignite and as it works ok on electricity, while my other gas appliances work ok, I suspect a sticking valve or possibly a defective flame-failure device.  I need to open it up and investigate.  The problem:  How to get at it?  It is inaccessible behind the Truma Ultraheat air heater.  Does anyone know whether it is normal to remove the air heater in order to gain access to the water heater?  I'd prefer to not interfere with the air heater's flue and gas connections.

When I had a Ravenna I had the same problem and it turned out to be a flat battery. It was an AA size I think and located at the bottom left of the heater. A bit of a fiddle to get at but heater worked fine afterwards.
Hope this helps.