I'm sure you will see Swallowtails in the area Rosalyn has detailed tomorrow. Good luck with the photos - they seem to permanently quiver, especially when it is sunny and they rarely sit still. If you walk out into the main Fen, keep your eyes open for flag irises along the rides - the butterflies like those to land on.
Number 1 tip though: You're going to need a
BIG floppy hat, plenty of water and factor 50 suntan lotion! - Good luck Susie
Trev