Well I was pleasantly surprised by WBAC, after hearing they try to reduce their valuation for any flaw they can possibly find I was expecting them to go over the car with a fine tooth comb but they really only gave the exterior of the car a minor look over and only reduced their valuation a tiny amount for the miles covered between their online valuation and the actual viewing. They seemed pretty keen to acquire the car and it was a very quick and painless experience.
Still a bit of a blow after the great trade in value in March (which was really the only reason I ordered a new car as it was too good to pass up on) and I now have the inconvenience of having no car for 3 weeks and have to travel 200 miles on public transport to collect the new car, whereas it would have been much easier to drive down in the old one and return home in the new one.
However after fees WBAC gave £2300 above the new Tesla valuation so I'm happy with that, still £2600 less than the March valuation, but that is 5 months ago now, the car has added 3000 miles, the new price has increased £1500, and I will now get a 72 plate instead of a 22, so all that factored in I think its still a pretty good deal.