I don't know why the other respondant is asking about the clock. It has nothing to do with the hot water.
Check that the hot water flow switch is not dropping out while the tap is running. It's unlikely but worth looking at. With the central heating off and cold, run the hot water. Feel the two large pipes (outside left and right as you look at the boiler)
These are the flow and return pipes to the central heating
they should not get hot when you run the hot water(maybe a little warm if you run it for a while but not hot)
If they are getting hot then your divertor valve may be faulty and allowing heat to pass away into the pipework and some of the closer rads. This can cause the boiler to short cycle.
Just as an after thought make sure that your boiler is a 105e not a 105e instant. If its the instant it will be the diapragm in the water flow switch..
T.H.S
April 2007