There's a button with a circle and a line in it. press that for 7 seconds and it should tell you the status of the boiler (error code basically). Press the mode button for 3 seconds to exit that status screen.
Apparently while I was out it was seen to flash F23. And we found the paper manual.
That is 'Maximum temperature difference reached between return and flow heating - water circulation fault - check the flow and return heating sensor connection - check the pump speed'
We've tried that a couple of times, and also tried putting into different modes for a while, with no success.
The burner is coming on for a while, and then shutting off again rather than remaining constant.
I doubt it could have been that it froze, we're in east London and it's not been that cold.
The model is a CaprizPlus.