I am in need of some pragmatic electrical advice.

I have a house, it has some electrical cable running through to an outside shed from the internal fuse box (20 amp or so), bog standard cable, not ideal but its lasted and wasn't done by me.

At the shed is another fuse box RCD breaker with 2 fuses one for lights, one for power.

I would like power into the bottom of the garden, so my thinking is...

1. Replace the outside RCD with a one with 4 RCD fuses
2. Use armoured cable into the 3rd fuse
3. run this down to the garden, digging it in a good few inches, perhaps laying in water pipe to be safer.
4. Rise this to an outside junction box with one going to lights (probably off a plug socket), and another going to a couple of power sockets.

Questions ?
What size armoured cable ?
Any major gotchas, I figure I could buy extra cable and run armoured to the new RCD box as that would improve the existing system feed.
Thoughts on a solution for in the garden ? I just want a waterproof set of sockets (which will be undercover) and the ability for some floodlighting to be hard wired in 2 sets 4 LED's left and 2 LED's right.

For just a shed in the garden it's usually a fairly light load so 6mm 3 core armoured cable is usually sufficient. If you want to build in some redundancy for future proofing then you could go to 10 sq mm 3 core.
Use the third core (usually yellow) as the earth wire. Do not rely on the armour for the earth.
I wouldn't bother to run it in a water pipe, just bury it with a warning tape laid above it.

Thanks £110 difference between the 6mm 200 plus and the 10mm 300 plus for 100m so as it will be rarely used and in the ground I think 6mm should be ok.

I figure somehow joint the light circuits with this
http://www.screwfix.com/p/ip55-enclosure-grey-270-x-180-x-135mm/86625

and then have the main socket outside

http://www.screwfix.com/p/bg-13a-2g-dp-switched-socket/67928

Firstly if you are swapping the consumer unit as youa re then it's Part P and it needs signing off and building control need to be involved...

6sqmm cable might not have sufficient current capacity to meet the regs in case of a fault, even if you use the armour as a PE. SWA can be buried direct, ideally 400mm down with a warning tape on top there is no need for conduit. Your best jointing system for outdoors are the vaarious epoxy filled splicing units.

The other thing to consider is what sort of earth system are you using. What sort of earth is the house on, has that been exported to the outbuilding and what about the rest of the garden. Or is the shed on it's own TT earth which makes life more interesting...

Good shout..

http://www.electricalsafetyfirst.org.uk/find-an-electrician/part-p/

A cable with live, neutral earth runs to the outbuilding, that's it. Any samples of the epoxy units you are thinking of ?