Rodney, here is the routine you should follow:
Is there gas available?
Are the isolation taps switched on?
Is the cowl cover removed?
Is 12v available and working?
Have you checked the fuse on the electronic unit? if blown, replace with a 1.25aAT fuse.
If yes to all above:
When you switch on does the red light come on straight away?
if it does come on straight away, you need to replace the electronic unit.
OR does it come on after about 6 seconds?
If it comes on after 6 seconds, when you switch it on first, can you hear the gas valve "clunk" as it engages?
Can you hear the spark ignitor clicking?
If you cannot hear the gas valve "Clunk" into life, it needs replacing.
If you cannot hear the spark ignitor clicking, then it is one of 3 things, but at this stage, you should get a Truma approved engineer to look at it.