all i know is if you want the parts the best place to go is adlink ( www.adlinkuk.com )
PAUL DICKENS, September 2008
sounds like divertor valve not shutting off ch supply when demanding hw.
basically valve should close ch supply to allow heated system water to heat your domestic water. if its not working properly you'll be able to feel all the heat going down ch pipe to rads and water will be luke warm at tap.