Thanks for the link! Looks like it's a regular problem...
It's a CALA house, and to be fair they seem very keen to get the issue sorted.
The windows and bottom window cill seem straight, I think my £5 pack of 6 paper blinds from Amazon might be making it look worse!
The bay seems to bend on the ceiling on the right hand side with a distinct bay-to-room slope angle.
The plasterer did originally try and say it was straight when it was more than 20mm out..
I think there's an issue with the original build here somewhere as all the other windows are completely plumb.
Cheers for the info.