Martin Cook wrote:

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

I shall assume that 4 and 5 are a single device - the 32A/30mA RCBO -
and that all the other circuits have no RCD protection.

That being the case you could install a suitably-rated MCB in position
10 (hopefully the busbar finger won't have been cut off) to feed your
new outdoor circuit. Then provide 30mA RCD protection somewhere
downstream - e.g. in a separate 2-module enclosure adjacent to the CU,
or by means of an 'RCD spur' unit elsewhere.

--
Andy