Service all booked for the 20th. Thankfully Horsham VW have agreed the diff should be serviced every 3 years, price quoted is ~£150. This hasn't been done on the car yet so might be the cause of the issue, will see if this resolves it. They offered alternatively to carry out an investigation and diagnostics before servicing to see if something else is faulty, at a cost of just over £100. Think I'll try the service first and see what happens with the fault code. My warranty doesn't cover servicible components, so anything diffy is off the table
Brake fluid change is also due, but they wouldn't price match anywhere else. They're charging £79 for it... A quick check on Halfords and Kwikfit and they charge £45-£50.
They also mentioned spark plugs are due every 4 years/40k miles, which also wasn't done by previous owner after 4 years. Again, can't imagine VW will be cheaper than anywhere else for this.