red light flashing on trumo heating control

We are running our Trumo Combi 4 E/6E water boiler heater on electric and gas and had thermostat set on 1 during the night as the cab was getting cold. Temperature outside was below 0. The heating cut out in the night and a red warning light started flashing on the right hand thermostat control. The manual gave a list of potential errors. We dismissed some of the suggestions (did not understand some of them)  and decided it had over-heated. The manual said to reset the over heating protection. In the manual, from what we could make out this is at the back of our heater and we could not get to it easily nor see the 'connection cover' to remove it and reset the button. In desperation we switched off the Sergeant main fuse box and then switched it back on. This did the trick and has continued to work OK but we have no idea exactly what happened or why the manual is so unclear. Any suggestions gratefully received.
Debbie