endymion wrote:

I see one or two of you may know something about storage heaters. Mine
are many years old. I do not plan on changing them all but I am pretty
sure the meter is being hiked by one particular one which is broken.

It's highly unlikely that a storage heater will fail in such a way as to
draw too much power. That would need a partial short-circuit in one of
the elements, which, if it did happen, would rapidly lead to self
destruction of the element, ending either in an open-circuit condition,
or a dead short across the supply (or to earth) and a blown fuse. It's
possible that the input charge thermostat could stick closed but you'd
notice that by a rise in the room temperature, or if the heater core
became too hot the overheat trip would operate, cutting off power to the
whole appliance.

Most storage heater faults are "not working" or "low-output" due to
open-circuit elements or operation of the overheat trip.

In an earlier post you said

If I extrapolate the figures for 90 days that would give a night
usage of 18473 units

That's an average of 205 kWh/day, or 8.5 kW continuous (24 hr) loading,
or 29 kW loading (120 amps!) over 7 hours. There's no way on earth that
a single faulty storage heater could account for consumption like that.

Other than it has to be 3.4 KW to replace the existing, has anyone any
recommendations for make?

There's probably not a lot to choose between different makes, their
construction tending to be similar. It is worth getting the 'automatic'
type though, with a room temperature sensing thermostat. Otherwise just
look for a good price. I've got two Unidare ones in my workshop
outbuilding. These were bought s/h (but only about one year old) and
have been trouble-free for the last 12 years or so, touch wood.

http://www.storageheater.co.uk/ might be of interest.

--
Andy