I currently have power to my garage which is fed from a 30amp fuse on the
main consumer unit to another fuse box in the garage. This fuse box has 2
feds one for a small socket ring (3 double sockets) and the other to
provide
lights for inside the garage and security lights.

Replace the garage fuse box with a modern consumer unit. You will want two
circuits. Firstly a lighting circuit fed from a 6A Type B MCB. Secondly a
radial (or ring) circuit for the sockets fed from an RCBO (B20A/30mA will do
here, although a 32A could be used if you can live with possible lack of
discrimination (the B32A type response may be quicker than the 30A cartridge
fuse anyway).

This will provide RCD protection to all the sockets. This is essential for
lawnmowers and power tools. The RCD will not cover the lights, so when you
chop through the mains lead of your handheld jigsaw, the lights won't go out
and you can stop cutting before you chop off your arm. You can run a
separate circuit for the shed if you like, but it is probably not necessary.

It may be best to run as a radial (with substantial cable, such as 6mm) as
it probably suits the topography better. Only one cable is required to enter
the shed from the garage on the socket side. (I assume the two are attached
to each other).

Christian.