Combi Boiler CH ok but no HW?

I have an old oil-fired Firebird C20 70K combi boiler, that provides CH and DHW to my house which has a single bathroom with sink, shower and bath. Lately the pressure and volume of the HW has been pretty risible...with literally just a dribble of water coming out - although the boiler fires up fine and there is no lock-out. Now this piddling flow has effectively stopped and there is no DHW at all. Indeed, in the upstairs bathroom there is definite hammering if the HW tap is turned on (although no hammering if the bath taps are run...but then, no water either:). The CH seems to work fine (so I am assuming that the problem is not with the heat exchanger in the boiler?). I have read other posts/threads which spoke of issues with diverters/diaphragms and sensing switches - given my 'symptoms' are there any engineers out there who could enlighten me - before I make the 'dreaded' call. And, before you all rush-in we are awaiting planning permission for an extension that will also include the removal of the combi and replacement with a sealed/unvented system, for which the cylinder and boiler have already been bought - if onlty the damn thing could have held-on for another couple of months.

Any help is much appreciated

Regards

Sam