We find a lot of different larvae while we're digging. This one was uncovered whilst digging a plot for potatoes.
It was very hard, like a dried mealworm we buy for birdfeed. It will turn into a click beetle. The type of beetle that leaps away with a loud click when you attempt to photograph it!
I'm glad I dropped him over the hedge (to the canal, not someone's garden!) as this click beetle (elateridae) larva eats roots and is particularly partial to potato tubers :-(