Update to Intermittent Hot Water Only.
Please note since I posted the original last year, I should point out that the other cause of this problem is the flow meter.
sometimes tapping this gently can start the boiler up for hot water. this is located on the right hand side and bottom of the boiler.
Also, a tip I was given for checking if the thermistors are faulty is to swap the two thermistors over. There is one for hot water and one heating. If you change them over the hot water problem should clear and the heating should have the problem. Then it is advisable to change both of the thermistors.
I hope this helps
Be sure to drain the water from the boiler and disconnect the electricity supply before attempting a repair.
Clive
May 2006