Apparently they were taken over by a company called Baxi. There's no mention of IMI Waterheating, but there are some contact details here - [broken link removed] - maybe they can point you in the right direction.
Combi boilers are brilliant when they work well, but there are so many components required to make them work well that diagnosing a problem can be difficult if you don't know the ropes; and if you're not sure how to fix it, you could make matters worse. Best option is getting a professional to look at the boiler.