yehr it sounds like your diverter valve is passing no real quick fix but to change it along with the diaphragm but from looking at my laptop parts your talking about £110 but remember only gas safe registered people are alowed to work on boilers

part number wise the manuf number for the 3way valve is 0008val09010/0 if u need ravenheats number its 01132 527007 for tec help they can always put u through to parts or to get a eng out poss