Snakes in the UK?

I live in southern England, not too far from London, and I am lucky enough to have a VERY large pond in my back garden. This morning I was hanging out the washing when a large snake just slithered across the path in front of me, and disappeared under one of the rocks near the pond! Now, of course, I thought it may be a gr snake (as we ALWAYS have these in the garden) but this snake was completely black (like 100%)...with just a small yellow line on top of its head! It was about 4 feet long, and was a VERY fast mover (much faster than a gr snake). Any ideas?