It doesn't matter what it says on the invoice, the invoice could be wrong. If you'd asked for 1 wheel and they'd sent you 100 wheels and put 100 on the invoice, that wouldn't make it right.
What matters is what you asked for in the first place. If you have a quote from them for 18" wheels then that is very strong evidence for what you asked for.
Don't get into sending them back yourself, tell the company to arrange to collect them. As Saraj said, keep evidence of all your contacts or attempted contacts - e.g. leave them a phone message if you can about this (and keep a log), try faxing them, emailing them, write a letter (and send it recorded delivery so you have evidence they received it).
Trading Standards will totally back you up.