Just to let you know what's happened about this. (This could be a long post!!)
I went to the First Choice Hypermarket in Nottingham yesterday and spoke to the manageress who said she could do nothing about online bookings as it was totally separate from the shops. This seems weird to me as it's the same company!

I came home and rang FC which proved to be a very long process! I was on the phone for 29 minutes and was transferred to 7 (yes, SEVEN!!!) different people and was put on hold each time!! Eventually I was delighted to be told that they would cancel my existing booking free of charge as long as I went back to the FC shop before close of business today and booked a fly drive instead. So DH & I went back to the FC shop today and were told that the flydrive prices for August would cost £1040 per person which is more than the flights only booking I've already got + a rental car booked direct with Alamo. The fact that we have to go in August (DH is a teacher) obviously makes the whole thing a lot more expensive!! So essentially I haven't saved any money but I am happy that I followed it up and found out all the ins and outs!! Strangely the brochure for Nov 2008 - October 2009 is in the shop but you can still only book flydrives online up to April 2009!

However we have got free dining so I can't complain. Thanks again for your help Kevin!