So a little bit of an update.
When I had the car on the ramps for MOT a few weeks back we noticed the O/S VSS cabling had worn, exposing copper wire.
This led me to get the OBD dongle back out and see what readings I get, and sure enough it was over reading significantly.
Since then I’ve bought a new sensor (£££ ouch) and fixed the cabling, but guess what, yep it still over reads!
I guess I need to get the cabling thoroughly checked now as it’s possible there’s an issue in the harness further downstream, but failing that I’m at a loss!