I'm not sure what an independent expert would cost, but assuming you win your case, I would expect these costs back. I would also have anticipated that in small claims court an independent expert report would suffice.
If the car has been serviced annually, with what appears to be a low mileage, there doesn't appear to be a reason why anyone else would have the plug off. Civil Court is based on the balance of probabilities, so
IF you can get an expert report to say that it shouldn't wear out after 13 uses without some misuse, which is what the dealer claims. Is it more probable that OP had it off for some other reason despite faithfully getting it serviced at the same dealer every year, and he damaged it, or that it was damaged by the people that take it out every year ?
There's obviously time & money at risk in going to this length, with no guarantee of return, I'm sure a reasonable argument could be had by the dealer but I can see why OP would be annoyed enough to pursue this, after faithfully returning to the same dealer to try to do the best for his car. It may also be worth the dealers time to settle rather than pay lawyers.
Other route, assuming they are signed up :
"Alternative Dispute Resolution involves an independent and impartial third party reviewing the evidence and making a decision, offering a view or helping the parties come to an agreement.
This is a free and swift alternative to taking the issue to court.
https://www.themotorombudsman.org/co...ke-a-complaint