When hot tap turned on water demand light comes on and burner ignites, water begins to get warm. Then after about 10 sconds both go off (unless central heating is on and then the burner stays on).
Is this a diverter valve problem? Is it curable or are we talking about new part?

TomUK, November 2012

I had the same problem on my 35 CDI boiler, the problem was the diaphram inside the water diveter valve was perished and had a hole in it. You can buy a new diaphram and you will need a "O" ring kit + 2 x 3/4" fibre washers (total £45.00)to complete the repair.
If you call out a heating engineer expect to be told you will have to replace the complete valve!! plus the O rings and labour around £250+. You do not need to replace the complete valve and you can do the job yourself if you have a bit of mechanical apptitude it takes about 1 hour, you can get the manual free on line. Hope that helps.

Roger N, November 2012

Yes diverter valve need change since you do that also clean water heat exchanger

ltd, November 2012