Its a 1958 onwards James Captain, (the front motor mount gives it away as a late model) James used Villiers or AMC engines I think yours is a 197cc so should have a Villiers engine.
The tank is incorrect to the bike.
Sorry to shatter any dreams of riches in your shed but an immaculate unrestored Captain worth maybe a little over £2000, Restored £1750 -£2000, in your condition £300 - £500 at a push with a v5
The original front mudguard is the most valuable bit as they can be hard to come by, so if that's lurking in your shed £150ish depending on condition
*Edit. I source/restore classic bikes for a living. I have provided private sale estimate prices.