"Any ideas as to how to book a ferry ticket on their web site if you don't know the car make/model/rego (we used Mrs QF WP's fathers old Rover details to get a quote)??"
Probably can't. The best suggestion I could make in this scenario would be to phone Wightlink and ask them for a quote.
Alternatively see if you can book on Red Funnel which runs from Southampton to Cowes which doesn't appear to ask for the registration number and save the problems.
www.redfunnel.co.uk
Dave