sounds like diverter but i'm sorry, i can't remember what type of valve it is on these.
Please be aware that these are long since obsolete - Worcester no longer produce spares for them. You may be lucky and find that the valve concerned is a part used on other boiler in which case you may be able to pick one up, but anything over and above that and you may have to kiss it goodbye.
Bunslinger, June 2009
