worcester 24cdi hot water problem

[topdec]

I am having problems with my hot water, it is not very hot when i run the bath and very cold when i run the shower (hansgrohe), if i run the hot tap on the hand basin in the bathroom it is hot and in the kitchen. It seems as if, the more demand required for hot water the colder it gets, i have been told it could be the diaphram in the diverter valve or the hot water thermister both of which i have had changed with no joy, i have recently noticed that the heating goes cold when the hot water is turned on, and the water is slightly warmer when the heating is off, could this problem be the vessel any help would be very gratefully received.
regards............mark.