Problem with myson Orion Fanfare Si electronic ignition?

Sometimes the boiler fires up fine (one or two clicks of the ignition) but other times it clicks away for a minute or two or much more then fires up but sounds the same either way ie not with a bang. Very occasionally I've noticed the boiler is running as if lit but isn't (gas just being blown straight out the flue). Fan runs fine (quiet) and boiler operates as it should other than that. Could it be the electronic ignition itself that needs replacing? Any advice welcome please!