I'm in the process of replacing my diverter too. You can get diverters cheap on ebay. Search for the Part number of "3 way valve" in the back of the Ravenheat instruction manual.
SiO
July 2009
Hi, I have the exact same problem with the exact same boiler...
I've been informed that it is a problem with the shunt valve (diverter)... also, the thermister on mine was pretty clogged up with a build up of scale.
I think a replacement valve (diverter) was around £160 mark...
fortunately, I cant get warm water when the central heating is on (enough for dishwashing) so I can wait until it get's worse I s'pose.