Truma Ultraheat

Normally I leave the Rienza on hook up whilst on the drive but as its in for MOT on Monday I have disconected the hook up lead.
Today I had to do a little job in there and decided to put the heater on. As I normally do I turned the heat knob and pushed it down to ignite the gas. There was no clicking sound from the heater so I tried changing the battery in the igniter unit. This didnt work. Before I pay a silly amount, £70-£80, for a new igniter unit I just thought it would be worth checking if any one has had the same problems without buying a new unit. 

I further to my last if it helps. The fan for the heater still works on all speeds.

Have you checked that there's no corrosion on the battery holder terminals?
Also check for any loose wires or connections. 
Not sure whether this video will help or not - it shows you how to remove and replace the igniter box but not how to test it:

https://www.youtube.com/watch?v=rVKAQfPGAzo

PS Just found this elsewhere -  Note I have not tried this, if you wish to then do so at your own risk:

"The best way to test an ignitor box is OFF the van ,use a 6 inch length of insulated wire ,wedge one end between the two fine pins of the connector and hold the other end close to the single terminal . Bam, BAM, BAM, if its ok. careful not to get a shock!!!."

Thanks Rodger, I did see the the video on You tube. As you said it helps to point out were the unit is and how to change the battery. I don’t like the idea of shorting things to test it’s a bit crude.
I am going to have another look tomorrow if it’s a bit warmer. By Monday night it will be back on hook up maybe it would be better left until then. 
If nothing else I cleaned out the fluff from inside the heater front.

I've had a problems with damp in the burn chamber, failed insulation on the thermocouple lead and three ignition boxes over the last nine years. The last time it didn't work I borrowed an refurbished ignition box from local mobile caravan engineers but it made no difference so returned it and book an appointment, tried it the next day and it worked fine and has ever since.

Everything Roger mentions is valid. Its easy to unclip the cover from the burner and see inside.

I have been out to Rienza this afternoon and the heater now lights.  The only think I can think of that might make a difference is that the leisure batteries were a bit low yesterday,. It has not been on hook up since I moved it off the drive on Wednesday and we have had grey overcast days until today. A mornings sun has pushed the voltage up to just over 13volts. Its not often its stood for this long. 
I had though of changing the leisure batteries and the solar controller this year so maybe this problem has proved it may be a good idea.
This enforced lack of use of our motorhomes has really showed problems for lots of owners. They are meant to be used not stood.
Hopefully this has solved the problem and thanks for the ideas.