the film I was thinking of was Shout at the Devil released 1976, set in WW1. The biplane used was in Portugese markings.

Ok found it…
BAPC.123, ZS-UHN, “P641”, Vickers FB.5 Gunbus replica, 1186/2, 1975, small components only remained – stored, Cranfield, b), stored 03.90 …

Vickers FB.5 Gunbus fsm. 1186/2. ZS-UHN. A.Topen. Cranfield. (Built IES Projects Ltd 1975). (As “P641”) (Built for “Shout at the Devil” film)

so perhaps it was a full scale mock up that I saw hung on the cinema frontage? Although it did have a South African reg.

Have a look half way down this webpage for “Shout at the Devil” Gunbus photos and video clip:

http://ships.bouwman.com/DH-1/Shout%20At%20The%20Devil.html