Boiler and Hot water not working properly-easy fix?

Hi,
My oil-filled heating system isn't working. I have an external Grant combi-boiler, and suddenly, 3 days before Christmas, the heating stops working. I still have hot water, but the temperature is fluctuating. It will get properly hot, but then gets luke warm after about 2 seconds, before getting hotter again.
I have my boiler serviced and repaired regularly (can't get in touch with my usual plumber), and have had a new Grundfos heating pump probably a year ago. I had a new burner maybe 18months ago.

Does anyone have any suggestions, or does this sound like a classic case of something or other, and is it likely to be something I can fix?