If it is Unmetered Supply, then the DNO for your area will (should) have a record for it. Most of them are for street lighting, a few are for bus shelters and the like.
The DNO is the company that owns the local network, it need not be the same as your Electricity Supplier (the company that sends your bill) but it will be mentioned at the bottom of your electricity bill. It will say something like
"Distributor:
Bloggins Electric Power Distribution, Some Road, Anytown, Countyshire, SW1A 1AA.
Call 0800 123 4567"
and there should be a phone number.
There will be a multi-box with a lot of numbers in it beginning with S nearby which relates to your own supply. It will not relate to the unmetered supply.
You might start by telling them there is an Unmetered Supply point in poor repair and the door is swinging open.
If you own the house it is possible that they owe you ten shillings a year for having it there.