Thanks for your response, AutoSlash. I've done a lot of rentals with Avis and things have nearly always been fine. I'm not sure emailing such higher management would actually yield a response.
I am still trying to reach the manager at Fort Lauderdale FLL, but it's a bit like trying to win the Florida lottery. I feel it's important to try and touch base with FLL, however, before going any further. Having made my reservation on the UK site of Avis I contacted customer services in the UK and they opened a "ticket" for me there, so the situation can be investigated. As for the credit card, no charges have shown up so far, presumably because the contract has not yet been closed.