Ravenheat LS100T combi boiler?

The boiler is not working on either the hot water or the central heating. When I go to switch it on the green lght comes on and will sound to be running for 10 seconds or more and then the red light comes on. If I turn the boiler of and back on again it will try again but will not ignite. I have changed the thermisters. Have tested the fan and is working ok Thanks Dave. I have checked the voltage to the gas valve coil and it is 12v ? when I turn the boiler on I also have tested the voltege accross the two connectors from the timer to the board if I test without timer connected to board I get 24v but when connected it disapears is this a fault on the board ?
Regards
Glynn