[ Remote brick built shed, originally wired up by a previous owner. Fed
via[/i][/color]

The shed is about 25 feet from the house. The total length of SWA/T&E
from CU to CU is probably around 20m or less


Proposed new set up:
3core 4mm SWA - earth is taken through the yellow wire - with the
armour sleeving bonded to the yellow wire at both ends.


Sorry, forgot to mention that - it's the standard TN-S with the earth
supplied by the electric co.


Two way CU in the shed with mcbs and protected by integral 30mA RCD.
Sockets (probably three) in the shed, either as a ring main or as a
radial circuit, plus an external socket mounted externally on the
shed wall for garden use.
- 2.5mm T&E fed from a 16A mcb

Shed lighting plus external light
- 1.25mm T&E fed from a 6A mcb




Not a lot - it's the odd bit of drilling or (electrical) soldering and
that's about it.

If it is an issue you could fit a battery backed emergency light (about
30 quid from TLC).

OK

In-house connection is to a 32A mcb on a split CU and is protected by
the integral 30mA RCD in the CU (there are two split CUs in the
house, this CU only feeds shower and outside lights, so if it trips
it's not a problem).

SWA terminated in the shed and indoors in a metal knockout box using
suitable glands. Metal box is also earthed (I assume that a plastic
box could be used instead if required?)

I realise that this means that the shed light will be lost if the RCD
trips in the shed. I could solve this by using a separate CU in the
shed for lighting only (I have a spare CU) or is this not permissible
as the light would still be lost if the 32A mcb tripped in the house
CU?


Battery backed light in the shed sounds like a whole lot less fuss.

--
Your original prposal with swa sounds ok dont forget to bond the sheath at both ends in your metal adaptable boxes to the yellow core used as earth ( needs yellow / green sleeve on that) If your swa is 4mm which is a 32A max cable you can also use 4mm twin and earth but the 6mm is ok, just that someone in the future may assume swa is 6mm as well. Th emergency light is called a non maintained eml, this only comes on with supply failure, go to any wholesaler and it should cost about £16.50.