If Agile is right diagnostically buy a new dhw flow valve part 562950
the Baxi part is probably cheaper
http://www.ezypart.co.uk/parts/part_id__2241
There is a service kit for the primary section and another for the dhw section but I don't recall part numbers phone Alpha ask help desk. As Tony says not easy. Here is the primary service section on ebay 260253823822
He might have a dhw flow valve section service kit for you if you ask
Believe it or not all I had as stock for that generation of alpha was the various servicekits the next generation I had both service kits and complete assemblies. In that next generation 9times out of 10 service kit was all that was required and actually easy to implement.
The one you have took me about an hour to completely overhaul from arriving at the house to driving away which is a very long time.
You have to thoroughly clean the insides of the manifolds.
You do see the primary section of this diverter valve on ebay here is one item no 360066839150
As for energy efficiency advice above your boiler is not a permanent pilot model. The only problems it ever has are the ones you describe if you bought new valves as Tony advised you would probably get another 10 years after which we will all be burning logs as there won't be any gas.. Actually I expect to be burning logs quite soon at todays gas prices. And i am going to knit my own solar.
The cost and management accountant in me does not buy the replace a band d with band a boiler argument. Anyone who falls for it is ignorant of capital project decision analysis.