I've come to the conclusion that the First Choice website is one of the most unhelpful I've ever come across! After extensive searching, the only reference to cancellations I can find simply states that if you pay the �25 online deposit, and then cancel, you have to pay the rest of the full deposit:
http://www.firstchoice.co.uk/sale/terms.html
That link makes reference to First Choice's 'standard terms and conditions' but they don't seem to have those terms and conditions available for people to read!
I suspect that, if you've only paid a discounted deposit and the additional money is referred to as a 'deposit top-up', you will have to pay the extra amount if you cancel. However, if the money you've paid is the full deposit and the extra amount completes the full payment, you'd probably only lose the �100 upon cancellation prior to the due date for the �600 payment.
I'm sorry that's a bit vague but, as I've stated, the First Choice website is far from helpful. (It makes reference to group bookings but doesn't allow you to search for them. Further, it won't permit any searches for June 2009; I'm surprised that they've accepted a booking that far ahead).
The only way to get a definite answer would seem to be to phone First choice or to send them an e-mail:
http://www.firstchoice.co.uk/info/aboutus/cont actus.html
Chris