[Andy131]

The vast majority of cars in the UK will have 300mm rear disks, these are solid - and rust?
Some have 325mm rear discs, these are vented - and still rust but less so.
Don't take the motor factors word for it, get down on your knees and look.
You will be "disappointed" if you spend an hour or two stripping down one side, winding back the caliper, removing the very stuck old disc to find out that you have been supplied the solid (much cheaper) smaller disc. Rebuilding the offending brake so that you can do it all again the next sunny weekend is also less than theraputic.

50,000 miles replaced the front pads - probably had 10,000 miles left, but it's a relatively easy job and the parts from the local motor factors are inexpensive at £50 (Delphi)
Rear pads were tired, still few thousand miles left, but the discs looked horrible. Brembo pads £40, vented discs £180
Less than £300 all in.
Compare this to LR at £800+ and local LR independent at £600+
Don't forget that it's 3 hours labour at whatever rate the garages charge these days.

 

[Andy131]

OK so I had the new vented 325mm discs as recommended by LR to the motor factors.
Stripped the RH side, compared the discs they looked the same so I tried to fit the new one turns out that the recommended disc is one for a Jaguar XF (part no. 09.C209.21), the Jaguar has 1/2" wheel studs, the DS has 9/16" wheel studs, or the metric equivalent. It nearly fits.
Getting good at this now.
The motor factors are at a loss as they contacted LR who gave them the part numbers - will try again on Monday.

If there is a moral to this it is don't buy an oddball model.
In the UK the D240 is far less common than the D150 or D180. Late 2018 they made a load of changes to the DS (Pathfinder diagnostics for example), and in 2019 it all changed to the DS mark 2 with very different brakes, mild hybrid option and styling changes

 

[ajmorr.is]

Oh dear! Hopefully you aren't paying for all the different parts? Good on you for doing it yourself though - brakes are easy if you have a driveway or some other space to work, and some basic tools.

ETA; I had almost the exact same problem on my former 5 series when buying front disks from motor factors, even using the VIN there were three different variations in thickness and diameter. At least they were al vented, which is easy to identify without needing to take the wheel off.