Worcester 28i junior will not ignite?

Hi,

I have the above model of boiler, when there is demand for dhw or heating
the boiler tries to light, you can hear it clicking, but gives up after 10 seconds. am i right in thinking it could be the thermistor, or is there a simpler fix. Have tried resetting it. there is a '1' & '2' button to the left of the heat selector knobs.

any help appreciated

regards