Glow worm 38cxi not lighting?

Have had the boiler for 8 years now and its been nothing but hassel ,about 6 months ago when I used the hot water it would run hot then cold it would also make reving sounds called a plumber out who said it was the thermister gave him the money to get the part never to be seen again,since then it wont light just revs so I have no hot water or heating called another plumber who fitted a new thermister and it made no difference he scratched his head for a couple of hours and said he would contact glow worm took the manual with him that was 3 weeks ago if anyone has a idea what the problem could be as it's starting to get colder now thanks