In reply to Lusk:
I have a glowworm and I'm mildly unimpressed with its reliability, very unimpressed with its serviceability and extremely unimpressed with the service its service agents offer.
I can easily reach my boiler but the glowworm service engineer was a shade under average height, result, a single rung step ladder would be required and (apparently) under working at height directives this meant the boiler could not be serviced had to be condemned.
Fortunately a local plumber equipped with arms that could be orientated upwards to shoulder height seemed ignorant of this aspect of HSE law and replaced the crappy plastic part that meant I had no heating mid-winter for two weeks.