Reckon you'll struggle. Field & Stream are mainly Game (shotgun) and fishing. Not really into rifles. Good shop and nice people.
Your best bet would be Sportsmans in Exeter on your way home. You'll be like a kid in a sweet shop, and you might even be lucky and find that they have your rail in stock.
Alternative on the way home is Braces (never been, but website looks good) in Bristol or Steve Beaty at Ivythorn Sporting in Street (top class bloke and shop, but you will need to call ahead to check he's open). Both short distances from the M5.