In uk.d-i-y, Paul wrote:

Thanks to all for your input! Sorry I forgot to mention that I have
the following unusual kitchen setup:-

- 30 amp ring circuit powering a microwave, fridge/freezer, boiler and
2 x under-unit lights
- 30 amp radial powering a cooker, toaster and kettle
- 20 amp circuit as mentioned before powering a washine machine,
tumble dryer and computer (the circuit goes up into my back bedroom to
power the computer!)

I would like to spur of the 20 amp circuit to my outdoor socket simply
because it is more convenient, although now it seems to be better to
use the 30 amp ring as it has less wattage on it.

Like I opined earlier, any of them will be "safe enough" if you use a
*fused* RCD connection unit. The 30A ring seems like the least loaded
circuit of the three, though I'm not quite sure what your "boiler" is
(a rice cooker? a Burco?); even if it's a 3kW jobbie, your 30A ring
will deliver 7.2kW, with the other appliances pulling under 2kW. So
that's the circuit I'd tap into. From the further details you give,
I'd firmly *not* use the 20A radial, as the w/mach and t/drier (which
could well both be on at the same time) will be eating maybe 4kW at
peak (though to be fair, w-machines spend relatively little of their
cycles heating water) leaving only 1 kW or so for your monster 4-way
Athlon-with-liquid-cooling computer setup ;-) and thus naff all for the
outside loads.

I have discovered that the 2 sockets at the end of the 20 amp circuit
I mentioned before are joined via plastic connector blocks which has 3
sets of wires (1 from the CU, 1 each for the sockets) - This doesn't
seem right to me (even more so because they are behind a blanking unit
right next to my sink). This still leaves me puzzled to why the CU has
2 wires on the 20 amp MCB.

It's possible that the sink has been moved at some point, or just that
someone got iffy about having a socket right next to the sink, as the
blanking plate+choc-box suggests that there used to be a socket in that
position. The two wires at the CU are indeed puzzling, and it'd be worth
tracing the Other one (since you know that just one of them feeds the
kitchen-and-back-bedroom run, right?) - initially by simply disconnecting
one of those two wires and seeing what, if anything, goes dead. Remember
it's still possible this 20A circuit is actually a ring, not a radial -
so the second wire could be the other part of the ring (maybe returning
from your back bedroom). If this is the case, you'd find that disconnecting
either wire alone would still leave all the sockets on the circuit live...
*and* the disconnected end live too - so don't be cavelier, and use a
multimeter to trace connections in preference to the mains supply!!
Or you might discover that the other wire supplies your rarely-used
immersion heater, in stark contravention of the Regs (an imm. heater
wants its *own* final circuit, not sharing with owt else, 'cos when it
switches on it pulls a Serious load for a Long Time, heating as it does
a rather greater volume of water than a kettle or a washing machine ;-)

Unfortunately I have no spare spaces in my consumer unit to fit a new
circuit.

Fairy Neuf - I see no tearing urgency to fit a new CU.

BTW - I will be protecting the outdoor socket by using a RCD adapter
and burying the cable in PVC conduit 45cm+ under ground.

When you say 'apapter', do you mean a plug-in jobbie made to go on the
end of a flex (not best practice for supplying a permanent circuit!), or
a nice accessory-box-mounted fused-and-RCD'd-spur-connection-unit thing
(the Right answer ;-)?

Do you think my best bet would be to spur of the 30 amp ring or
install a new consumer unit either with more MCB's or in addition to
my current CU?

For occasional use of kit outside, I think tapping into the 30A ring is
quite acceptable, given the constraints on your existing CU. At some
point, depending on time, resources, and other changes you want to make
to the installation, a larger (more ways) CU may figure in your plans, but
it seems like a 'nice to have' rather than a 'must do' on the information
you've given so far.

HTH, Stfeek (or agrnaam thereof)