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Sqad, my French doors are located in Wiltshire, England, United Kingdom!
I didn't think it was a grass snake, much smaller. It almost looked like a worm, no pronounced head to it, and almost black, not the look of a normal garden worm. It was the 'sidewinder' movement of it that threw me - only ever seen that on television before!
Back tracking, I suppose it could have been a grass snake, but a small one. The 'sidewinder' action could have been because it was moving over dry timber decking, whereas I have only seen grass snakes before moving through ... well - grass!