Worcester 24 CDI RSF Boiler Problem
 

I have had a Worcester 24 CDI RSF boiler now for 4 years. It has functioned well over this time, but for some reason the domestic hot water has stopped being heated.
Water flow appears to be normal, the red hot water light comes ON and the boiler fires up but the water coming out of the tap is cold. The diverter valve appears to throw over and can be seen through a small aperature.
It appears as if the valve behind the domestic hot water heat exchanger is not by passing the boiled water into the domestic hot water heat exchanger.
The boiler works perfectly in the central heating mode.
Any suggestions would be most welcome.