Question Author
hi steve
thanks for your input mate. i got in touch with aristons technical earlier and their engineer was adamant that it can't be anything other than sludge in the system and advised me to take out and clean the main and secondary heat exchangers. Can't say i particulary relish the thought of that as the styx is a bag of crap and really difficult to access anything, they also seem prone to the joints rusting in, and whenever you force a joint open it invaribaly opens up another joint further down the line leading to a leak.
my father has had a similiar problem before and he also says it could be a pump problem. apprently the styx works like no other boiler on the market and the pump pressure needs to be greater on the water side than on the heating side, so even though the pump seems to be working fine on the heating it could not be running efficiently enough to fire the hot water.