Truma 300 gas heater?

manual ignition Truma 300 Tested earlier and passed:
. I had it going fine for the first day, now the pilot will not stay on even when cotrol knob is held down for 40secs, twice the recomended time, is it thermocoupler fault?.
How are the side bottom clips release to remove casing to access and to fit new coupler.

For the sceptics: Yes I am competent having been instructed by my brother who was a corgi reg engineer.