If the Barn Shop is not convenient ... you can go to The Country Store in Pampisford, near Sawston. Generally 'while you wait' service.
To find them: M11 Duxford exit. If coming south, exit left to Cambridge / Newmarket A505. Continue across a small roundabout and turn left at the large roundabout (BP garage on your right). Then immediately bear right. Follow about 400 yards and turn left into the industrial estate .. you can see the Country Store from the road ... it is immediately in front of you as you enter off the road. In the Store, right door into the 'Gun Room'.
Cheers, Phil