worcester bosch 24i intermittent ignition?

Hi,

My WB24i has an intermittent ignition fault developed over the last few weeks.

the DHW demand seems ignite the boiler with no problem.

the CH demand however will USUALLY ignite the boiler on its first attempt, however as the CH begins to "cycle" on and off with water temp, it struggles to relight the boiler at each cycle.

the "ticking" of the spark can be heard but many times not seen. the boiler locks out after several seconds of "ticking" without ignition, forcing a reset by the off switch or ceasing CH demand.

the spark electrodes were changed for new ones this morning and set to distances specified in the manual. with no difference in effect.

gas jet pressure was also checked and tweaked this morning too. also entire ignition chamber was swept out and cleaned.

anyone any ideas? thanks in advance

Steve