"The Question Asker" wrote
| Ok, obviously running a new 3 core cable is best and not so
| bodgit but is it ok to just run a earth cable.

Not really. What we're talking about here is not just 'earth bonding' but
also circuit protective conductor (cpc) which *MUST* be run be run alongside
or as part of all the circuit cables and to all the accessory positions.

If you put in your earth cable you might meet the need for the bonding of
the metal faceplates to earth, but it completely ignores the requirements in
the Regs for cpc. Your circuit would remain non-compliant and, ultimately,
require rewiring.

Feeding cables down the switch drops - if conduit has been used then feed
the new in as you pull out the old. If conduit has not been used then it's a
redecorating job. You can sometimes use surface cable to switches, or
ceiling mounted pull-cord switches, as temporary measures to avoid immediate
redecoration.

You might as well take the opportunity to improve the provision of lighting
points, eg split onto upstairs/downstairs circuit, put in wall lights or 5A
sockets for table lamps switched from the door in the lounge, two-way
switching at the bedhead for bedhead and room lights in the bedrooms,
interconnecting smoke alarms, etc.

Owain