Thanks for the reply.
Shift F1 wasn't it but it turns out that I had a conflict mapping for gunsight view (in turret) and loosen shoulder straps. Even though remapping them yesterday didn't work, but I had all sorts of problems with my key configs, they kept getting deleted by Steam Cloud. It works now. I still couldn't get the Blenheim turret to work, but it appears it only works when you have taken off.
Why couldn't someone just tell me it works for them....? Nevertheless there's a bug with the key assignments here though. 1 they don't always warn of a conflict when it matters, and 2 changing them doesn't seem to fix the issue until you restart the game.