Right thanks. Please could someone point me at the appropriate place to read up on R vs Z? I've tried to find information but it's hard to search for "R" and "Z" and most of what I've found is unhelpful AC pages talking about it without explaining what the differences are!

I guess one advantage, is Lat. being fully refundable along with all tickets within 24 hours means I can always book, try to upgrade then cancel and re-book with flex/tango if nothing is available.