I've always wondered about the 50 ft rule.

I plink in my garage on ocassion, with air pistols. Its within 50 ft of the road, but obviously no one in the right mind could be endangered, interrupted or alarmed. (the garage door is shut)

I think the same must go for back garden shooting. Is it even possible that someone on the pavement at the front of your house could be "alarmed" by you shooting in your back garden? Even Walter the Softie ought to be OK.