Truma water heater not working on gas

UPDATE- 
Turned to the age old solution from my IT days of turning it off then on again.
Turned everything off. Turned it back on again and hey presto. Water coming thr=ough hot on gas.
No idea why, nor do I care... it works

Thank you for all your suggesyions.

Apologies for another question.
I've tried to test the hot water. After clearing an air lock, I cannot get the Truma water heater to produce hot water. It DOES work on electric mains.

I have checked gas is on.
I have lit stove to make sure gas runs through.
I have run hot water tap without turning on water heater (runs smoothly)
I turned on the Control panel to gas and set temperature to 70 degrees
Green light shows on Control panel
No red warning light is showing.
Water runs cold.

When I purchased it, the owner demonstrated the hot water using gas so can only assuming I'm doing something wrong.

Any advice would be greatly appreciated.

Have you removed the flue cover outside. I'm assuming you have Truma Ultrastore and not a Combi.
The Ultrastore cover looks like this...

Paulmold wrote:Have you removed the flue cover outside. I'm assuming you have Truma Ultrastore and not a Combi.
The Ultrastore cover looks like this...

Yes, I did that. And it is not the Combi

We have a similar water heater.  If it fails to light on gas a red LED glows after a couple of minutes instead of the green.

Touch wood I've never had a problem with this equipment so not had to investigate any further.  Good luck in getting it sorted.