Thank you all for your advice. I decided to get the bus and Gatwick Express this time (I have a Young Persons Railcard, so the fare is reasonable). It took ages, but it was easier than I was expecting. I'll be making the journey again at some other ridiculous time in the morning in December and also returning on Boxing Day, so Butterfly Cabs will probably be hearing from me then - if they're even open! : ) Thanks again.