Probably the diaphragm in the diverter valve. This part can be bought for about a tenner. But there are a number of things that it could be....Does the boiler fire up on demand for hot water? i have known microswitches to be faulty on this model. What is the flow rate of water at the hot tap? if the flow is reduced then the microswitch might not be made and therefore won't switch the boiler on. Do you live in a hard water area with limescale? If the plate (water to water) heat exchanger is full of limescale then the heat transfer might not be sufficient from the primary side of the boiler. There is a reasonable fault diagnosis chart in the back of the "Installation and servicing" booklet that comes with the boiler(available from the Worcester website) that is quite simple to follow.
the boiler expert
June 2008
if its not firing when the tap is opened, its either a partially blocked in-line water filter or dhw flow sensor is scaled or faulty or last but not least the dhw thermister maybe faulty
as suggested below, you would look to the diverter valve if the boiler was firing when tap is opened but either you were getting luke warm water with luke warm rads, or only hot rads and cold water
T
June 2008
could be the water divertor valve, in particular the diaphram
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