Went and checked the car out yesterday. They did a service on it before we got there - oil, filters, new front brake pads. Bit of a lip on all four discs and some surface rust but nothing major so expect they were the originals. Tyres looked pretty much new, loads of tread left and in good condition. Clean and dry under the bonnet too, no signs of fluids leaking before and after test drive.
Test drive was good, although being in West London with traffic everywhere we couldn't get above 40mph. Rear light bulb had gone which they somehow missed but they changed it for us on the spot, ending up buying it for £8,700 with 6 months warranty.
Mum drove it home from London to Stratford upon Avon up the M40, she said it was making an odd sound like heavy rain hitting paper from around 50mph and upwards. No vibrations through the car or wheel and she pulled over on the hard shoulder to check for anything obvious, but being dark and a motorway she carried on home and will get it checked by a local mechanic today.
Google Gemini suggests it could be a loose wheel arch cover, door/window seal, or water collected behind a panel somewhere so hopefully it'll be straightforward to fix.
