Thanks for the advice guys, I have since installed a 3rd card (decent enough PSU finally arrived) and this pushed temperatures up even more, though I overcame the restricted airflow problem with a very no-frills solution...http://img593.imageshack.us/img593/5697/20110625171440.th.jpg
Wedging a wider gap has helped loads now, no more 80C+ temperatures. The cards are running at:
66C [960MHz core, 320MHz mem] = 425MHash/s
72C [950MHz core, 316MHz mem] = 420MHash/s
78C [905MHz core, 301MHz mem] = 400MHash/s
To be honest I was only aiming for 1GHash/s, but the bit of MacGyver has boosted me to 1.25GHash/s - well chuffed!