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How do I fix my Worcester Heatslave 20/25 combi?

If I run the hot water for a few minutes the temp drops to about 20 degrees, I've turned up the dhw thermostat but this only makes the water hotter for those first few minutes. What else can I do?
Andy, October 2013
It's oil fired so I can't tell.

Andy, October 2013
can you see the burner flame going high then low then high again

A1, October 2013
link Click here to see other fixes for Worcester.