You can have a sub mains supply taken from the existing head end. This
entails only three Henley Blocks and one mains isolator switch to be
installed so that both supplies are still taken through the original
electricity supply system. The same applies to this as with the gas. The
same supply was always used by the whole house anyway, so just separating
the house in to two flats should be just like running the whole house as
before.

One note on this - this used to be common practice, but it is becoming
less-so now, and there are technical reasons that go hand-in-hand with
this.

If the supply is in another flat, they would have no independant means of
isolation, and the "new" supply would require a seperate meter point
administration number (MPAN) creating for it.

Getting an MPAN is like pulling hens' teeth, and may consist of several
components that only an electricity co. may be able to provide, such as
the load characteristics of the proposed use, and tariff information.

A separate electricity meter must be fitted by the Leccy company contractors
of course, but you can run a 25 mm Two Core SWA (Steel Wire Armoured) Cable
yourself from the existing head end to where you want the new meter to be
sited in the new flat. You can also install the Henley Blocks and a main
100 Amps rated isolator switch, it doesn't need to be fused because there is
already a fuse on the existing main head end that it is all connecting to.
The Leccy co. contractors will do the rest for you if everything passes
their final tests.

Any submain over (IIRC) about 2 metres should have a fused isolator -
you're not supposed to rely on the fuse in the electric companies'
equipment. The fuses in this isolator should be at a lower rating than
the fuses in the electric co's equipment to provide discrimination (in
other words, your fuses blow first)

There may also be a legal aspect to the electric co. fitting a meter to
the end of a submain - they may be seen as accepting responsibility to
repair the cable if it should fail at some point in the future. On a
safety related note, their jointers are not allowed to work on cable for
which they have no documentation.

There are some anomalies as far as submains are concerned, but these (to
my knowledge) all relate to rising mains in large offices / buildings.
The rising main belongs to the building owners, and it is down to them to
maintain it. In order to get a supply to a newly seperated part of the
building, their own electricians fit a tap-off box to the rising main,
lay a submain to the new location, and will typically fit their own cable
head / cutout.

As far as tests are concerned, test-notes used to be requested for new
supplies (in our area at least), but these are now being phased out - the
electricity co. warrants that the installation that belongs to *them* has
been installed in accordance with regs, but anything beyond that point on
the customers side is the *customers* problem. A new installation may
typically be terminated into a double pole isolator, and the customer has
to get their own electrician to connect their side to the supply.

The electric co. does have the ultimate sanction of being legally
required to render a supply safe if its in a dangerous condition, but
that may be by simply removing the main fuse, or in extreme cases
(vandalised / vacant / fire damaged properties) by disconnecting the
supply externally.

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