ravenheat rsf 820 fault?

hi guys, ive got a problem with my ravenheat boiler,it fires up perfect for central heating but only lately when i turn a tap on it doesnt fire up untill i put the tap on full?
ive also noticed in my bath what appears to be little black bits which lead me to suspect a rubber diaphram or seal has gone and theres bits coming out,i suspect its the diaphram in the diverter valve but was wondering if anyone else could suspect anything else before i go and order one of these,its worked perfect since we moved in the house apart from the pilot light going off every once in a while.
i would appreciatte any replies as i cant really afford to get a boiler man out but i feel confident enough to tackle this as ive fixed a few other boilers in the past?