They also used the Bristol Blenheim Mk.I from Duxford and a Yak-52 that has been modified to serve as a Spit lookalike for inflight cockpit shots. More details here:
Scramble Messageboard ? View topic - Filming at Urk, aircraft at EHLE? and on the Keypublishing forums (see links on Scramble board, Keypublishing is currently down for maintenance).