Thanks. This is a great help.

I have had a look again at my Consumer Unit. Its a Crabtree
Starbreaker (1995 vintage) with a capacity of ten modules, nine are in
use.

It is laid out as follows, left to right:

1/2 Main switch (100A) - occupies 2 modules.
3 MCB B32 - Cooker
4 RCBO B32 - Downstairs Ring Main + Garage/Pond Spur
5 30mA Device (trips RCBO32 when test button pressed)
6 MCB B32 - Upstairs Ring Main
7 MCB B16 - Central Heating
8 MCB B6 - Downstairs Lighting Circuit
9 MCB B6 - Upstairs Lighting Circuit
10 Blanking Plate

Martin

I would be looking at getting a dedicated supply down to the garage
and pond.
If the existing CU is full then as you only need a 13A supply I would
remove one of the lower powered circuits from the existing consumer
unit on the non-RCD side.
Use this circuit, (with a 32A MCB fitted to it), to run a 6mm cable to
a 2 way consumer unit with main switch incomer next to the existing
one. Populate this 2 way CU with a MCB rated for the circuit you just
removed and a 16A RCBO for the outside circuit. If 16A is over-rated
for the cable that is running outside then fit a switched fused spur
between the RCBO and the supply going outside.
The other alternative is to fit an RCD onto an existing non-RCD'd
circuit to feed the outside.
The above assumes you have a split load consumer unit, if it is one
RCD supplying the whole installation then you're going to have to
change it or add another consumer unit with RCD for the outside
circuit, wired into a Henley block between the existing CU and meter
position.