Problem with Heating on Sime friendly format 80 e?

I have just noticede today that the temperature control on my boiler is no longer working. It seems to have set itself to low and will not let me increase to heat the house. I will fire up for a minute or so every so often when I have it turned up full and adjusting the temperature makes no difference. The hot water is fine and it will fire up when turning on the hot tap.

Any ideas how to fix this?