Watched the film again last night.
The camera shots in the dogfighting sequences (pointing from the 'Spitfire' towards either the 'He111' or the 'Me109') showed the 'Spitfire' in question wasn't a spitfire (it was a Yak I believe).
Nolan wanted to actually show the Spitfire pilots 'in the air' during the aerial and so converted the rear cockpit of a Yak (to make it look like a Spitfire) to make it look so.
Its a shame he couldn't have used one of the many two seat Spitfires around to do this. Indeed the Aircraft Restoration Co (who provided and maintained the flying aircraft) have their own 2 seater.
I guess the practicalities made it difficult-either that or they were making too much dosh flying well heeled members of the public.