Was trying to avoid another call out, but still have three years left on the warranty so will. re adjusting, I've opened the flap and pressed both top buttons and the screen just lights up. Have a feeling that to get the menu you press both top buttons together and then can scroll though, but don't want to f anything up. That's why I asked in the OP if anyone could guide me through.Can't see why you can't at least access the d codes as there are numerous posts re the householders changing the parameters including d.0, it must be relatively simple to do this and allowed by vaillant.
If the boiler wasn't serviced since the extended warranty work (PCB etc) then they should return to fix it and even if it has been serviced except they find something seriously set wrong should still honour the warranty.
Why not, at least, give them a buzz?.