I picked up the April 2015 copy of Sporting Gun and it answers earlier speculation, (see above), about Napier of London (makers of gun care products), perhaps once being gunmakers:

It seems Steve Rowe, the founder and managing director of Napier of London was London born, but his family emigrated to New Zealand when he was a toddler. He's since returned and "Steve launched his company, naming it after the New Zealand city of Napier where he grew up".

Interestingly, Rowe explains that if you put a wet gun in a slip, it starts rusting almost immediately, and it should be rubbed down and dried as much as possible first. His claim is that if the metalwork and bore are then coated with a patch of their product VP90, (which comes in handy sachets), "the corrosion inhibitors in the oil start working almost immediately to prevent rust". Since it's raining today and I have a pheasant hunt lined up, I'll give it a go.

It's quite possible that, the 'Napier' name on this old shotgun is simply a retailer and perhaps not from Great Britain at all. We often see here in New Zealand shotgun ribs marked, "Tisdalls of Auckland", for instance. I'd be fairly certain this company never made a gun though they must have imported and sold a great many in this country with their name already stamped on them.