The cast-iron head is due for replacement. There is a significant risk of personal injury if anything goes wrong there, so it should not be disturbed by anyone not wearing arc-flash protective
clothing (and it is also in theory an offence to interfere with the DNO's equipment unless authorised.) As your spark mentioned, the live terminals at the bottom are connected to the public supply, if they are shorted out it will be the entire street or neighbourhood that goes black and molten metal will be sprayed in the face of whoever does the shorting. It should all be enclosed and sealed, and these days preferably replaced with an all-plastic unit. Advise the DNO and they will probably be quite happy to upgrade.
The meters do not look very spooky to me, just ordinary meters. One is presumably redundant, the other ought to be metering the supply to the house but evidently isn't. There's nothing mystical, the speed of rotation should be exactly proportional to the load; two 2kW heaters should make it go twice as fast as one, and with the main switch off it should stop completely. If it does not, then either it is faulty in some peculiar way, or it is powering something you don't know about and registering the usage accordingly. In the first case, it is the Energy Supplier's problem to rectify, or they will send their appointed Meter Operator to do it. I have no idea of the liabilities involved if the estimated account is far adrift from the meter reading. If the meter is registering a legitimate load (such as a heater running in a shed that has been on for years) then it might get a bit more tricky as the customer will be relying on their clemency to cancel some or all of the bill. If they have been lax in checking the meter or requesting customer readings, there might be hope.
How large might the bill be? You say 26 seconds per revolution. A meter of that type might typically make 150 revs/kWh. Take the current price of electricity as 14p/kWh.
3600/(26 x 150)= 0.92kW
0.92 x 0.14 x 24 x 365 = £1128 per year (if 150 rev/kWh meter)
But, if the recording train is not turning and registering on the dials, then it might be zero. All a bit vague so you'll have to bite the bullet. Might be worth checking the reading as it stands, with the estimate on the bill, to get an idea of what to say.
And, for the record, this is an old and spooky electricity meter. Actually, not spooky either, just a very clever piece of engineering from about 120 years ago that is still working perfectly today.
