[Monkey Orchid]

Looks quite like a Grey Rat Snake (or Gray Rat Snake, if you want to use the American spelling).

Apparently these usually grow to a length of 3 - 5 feet, although specimens have been recorded at 6 feet. They're not venomous but can bite if handled (they also vent foul smelling liquid as a defence, much like Grass Snakes do). They're good climbers and also burrow underground, and prey on rodents, birds, lizards and frogs.

I'm impressed at your cool in getting these photos - like NickCantle, I would've been extremely wary if I'd come across a big unknown like this on one of my walks!

I wonder if it'll survive the UK winter? According to the Internet, their native range is the southeastern United States.