Problem with hot water. C/H OK but when any hot tap is turned on, the boiler fires up for a couple of minutes and then cuts out for about 30 seconds and then fires up again. Worcester have been out and changed the heat exchanger ( 230 Quid !! ) which seemed to work for a few weeks at least, but has now after only a few months reverted to on off mode again. Have been advised that the heat exchanger could be sludged up and requires flushing ???