potterton suprema boiler won't come on?

Potterton supreme boiler won't come on after being off during the summer. The indicator light just slowly flashes orange. Also I can hear a very faint single clicking sound which repeats after a gap of a few seconds as though something is trying to turn on. This boiler is used with a gledhill boilermate and l can hear the pumps are working in the boilermate. Does someone know what the problem is likely to be please?

Polly
December 2014