It has an external catch but it's not the one sold as 'Aston', which is much bigger externally and looks great when fitted but spend spend spend

I believe that this is 'ours'.
Classic Alloy 3 in (76mm) Locking Filler Cap
http://www.holden.co.uk/displayproduct.a...p;pCode=015.374

This is the best Aston:
Aston 3 1/2 in Alloy with Roller Catch (89mm)
http://www.holden.co.uk/displayproduct.a...p;pCode=015.193

Other smaller Astons are available........................

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4