No DHW on a Alpha Ocean 240 Eco?

My Ocean Alpha 240 eco combi boiler struggles to produce any DHW. You can only turn a bath tap 1/4 to half turn max to get it to run a bath. This is a condition that has got worse over time.

I assumed this was the diaphragm in the divertor valve, but on inspection it seem both plungers are coming out the full length of their travel and actuating the relevant switches. So, just incase I took the switches of and manually operated them but to no avail.

What happens is the boiler fires up as the DHW runs but just as it starts to get hot it cuts off. So I dont think its the diaphragms and/or switches? What is the likely cause of this? Manual here if it helps.
http://www.alpha-boilers.com/product...vice_guide.pdf

Dave Jackson
April 2008