Had a delivery of parts today from Car Builder Solutions.
They've sorted the parts to connect the BMW petrol filler neck to a new cap.
I'll list the part numbers as they may be useful to others.
The cap I chose was AST100, an Aston style locking flip top.
Other parts were
FFF51- ULTRA FLEXIBLE FUEL FILLER HOSE 51mm (2 feet)
POLY - BLACK POLYURETHANE ADHESIVE/SEALANT
ALIT50 - 50mm OD ALUMINIUM TUBE 0.19metre. beaded at one end
H45ZN - 2x HOSE CLIP 45-60mm
Cheers, Bob.