Thanks for your replies everyone. I think I'll go for it with the twin seats on the A380, but just reserve the regular seats on the B777 - they're only £20 each so not as bad. I don't want to just chance it at online check-in, because we probably won't be able to check in online until 24 hours before the flights, so everyone else will have had a 24 hour head-start on us. The actual seats may not matter so much, but I do want us to be able to sit together.