Hi it could be a couple of things yes Ravenheat do make crap boilers is there any chance u know the gas councle number of the boiler? should start with 47 then 5 more numbers

couple of things if your heating is off and cold and u run the hot tap does the heating flow pipe become hot if so its possibly the diverter valve passing its quicker easyer and cheaper just to change it if not it could be the plate heat exchanger scaled up its rather hard to diagnose over the internet

good luck with getting it fixed