Jo, perhaps you could do what I did as I had exactly the same problem. Book the itinerary that the system is offering and keep checking back - when it shows the itinerary you want, call Customer Services and see what they will do for you. If the worst comes to the worst and they want to charge a horrendous change fee, book the second ticket, cancel the first and argue the toss about the cancellation fee. They waived mine so would most probably do the same for you. Let us know how you get on!