I decided to include an application for an explosive certificate as part of my recent FAC renewal and consequently sourced a compliant wooden box for storage of 777 which I use quite a lot in my BP revolvers. The FEO visit went smoothly and the FEO couldn't have been more helpful while taking a look at my storage cupboard and box. The certificate was free of charge and will cover the purchase and storage of my powders without too much additional effort.
Maybe all police forces are not so sympathetic or helpful, but my relations with Sussex Police have always been friendly and efficient.

Brian