While looking at the possibilities for rewiring for the freezer I discovered three cables running through the garage from the consumer unit and disappearing through the back wall. One is an armoured cable which I have identified as the power supply to the garden shed.
The other two appear to go underground in some plastic trunking going I know not where. They appear to be standard ring main cables but I haven't a clue what they feed. I just hope we're not supplying a neighbour with some of their power ;o(