I think you will find the mouldings are hollow, so cutting/sanding will look a real "bodge" job - in my opinion. Far better to find a local UPVC supplier and get them to supply a flat panel - far less hassle.
I did wonder about that ...
I see from your location that you're based in Derbyshire - I don't suppose you have any recommendations (I'm in Sheffield, so it's not too far away)? I emailed some companies to ask whether they sell the sort of door panels I'm looking for but haven't had a single reply so far!
Cheers,
Alex