Valliant TurboMax Question?

I've been trying to stop this boiler from being on too often in the warm months. House is baking at present, yet the heating keeps kicking over as if the pipes were freezing or something. Had a go at the diagnostics thing, but the values I got up (winter mode=1) didn't seem to be adjustable.

My heating bills are pretty huge.

System is bled, only water (clear at that) coming out of the pipes. The system has been fernoxed.

I've probably missed something in the manual, but any help would be appreciated.