Well it turns out that you can still get a VAG part. I ordered a RHF arch trim from my local VW dealer.
Part numbers are
191 853 717 C - left front - £20.93
191 853 718 C - right front - £20.93
I presume you can still get the rears as well.
All went okay fitting the new arch apart from the rust down near the bottom of the arch (as circled in red on the pic).

The very last rivet in that area couldn't go into it's original hole due to corrosion so we had to drill a new hole in the arch trim and to the car body and stuck the last rivet in.