Well done, that is the right thing to do.I took it back to Halfords and got a refund in the end. I will buy one that IS the same size to be safe.
I showed them the photo at the till, although they accept return of purchased items within 30 days even if unused anyway. The one I wanted wasn't in stock but I can get one elsewhere, no rush, I intend to do it after the MoT in July.
Please accept my apologies, on behalf of all the good contributors on here. Your cry for help appears to have been hijacked by somoeone with poor advice, which you have ignored. Thank you. The standard of help given on this forum is usually to a very high standard.