About half way along the trench I hit metal, and found the above. I imagine it's an ancient plough that has somehow become buried with time. It was too big to dig out, so I've left it there for some future amateur archaeologist to re-discover.
When we recently up-dated our kitchen, the idiot plumber managed to clog-up the exterior waste pipe with gravel, so we are having to create a by-pass with a new pipe; this involves us hand digging a long shallow trench about 2ft deep.
When I draw up the map of the new pipe position (for future generations), I shall include reference to an 'unknown, non-ticking, metal object'. Nothing like a bit of mystery!