> 3. Extending a ring where I want to mount two additional mains sockets
> next to an existing one (can't use spurs as an additional one is
> For (3), can I use chocolate block in the back boxes of the existing
> and new sockets to extend the ring? Presumably it needs to be wrapped
> in insulation tape as the box is earthed? If this is allowed, then
> it's actually a better solution than what I originally proposed, as
> there is no need to chisel a hole in the wall for the junction box.
It's a win all round to do the cable joining only in the backboxes,
where possible - saves you/future occupant hunting for the junction
boxes when repairing or extending.
As Owain says, you don't need insulting tape round the chockenblocken -
they provide adequate insulation in their own construction, and the
adhseive on tape usually goos out over the years, making a Mess. One
thing which can help - depending on house construction - is to replace
your existing backboxes with deeper ones. (There are 3
generally-available depths: a plaster-depth one (about 12mm/half inch) -
mainly for lightswitches, which are shallow, and block or brick
construction with a thickish plaster skim keeps the whole of the box
under plaster; the 'normal' 25mm/1inch you'll see with most 13A sockets,
FCUs, and friends; and a 38mm/inch-n-half depth, classically used for
cooker control units, 45A switches, and other heftier gear with heftier
cables. If you can make room for a 38mm in place of the existing 25mm,
you'll have more room to work.
Then you can fill that room either with chocblocks as you suggest, or by
taking two spurs off the back of one socket. Provided the two spurs
won't both be likely to be heavily loaded (and the socket terminals are
big enough to take 4 conductors and make good contact among all of them:
MK, Crabtree should be fine here, pound-shop-special-pack-of-three not!)
this is quite permissible. Where all the sockets are in the same room -
even more so if they're on the same stretch of wall - it's most unlikely
that there'll be an excessive point load on the ring.
It's also possible to keep things on the ring by using two singles
side-by-side in a dual, as opposed to double, mounting box. One
original cable runs to the LH single, new cable to new socket, new cable
from there direct to second new socket, new cable back to RH single,
original cable joins at that RH single. Presto, all on ring, no more
than two cables in any one socket. (Not my idea, this - suggested on
this group a coupla weeks ago, too useful not to pass on ;-) Or you
could do 'half' of this with only 1 choccie block - keep 1 original
cable in original socket, go out to your two new ones from there (i.e.
existing-socket-to-new-socket-1, on from there to new-socket-2), and
bring back a new link to the existing socket position and join to the
other existing ring cable in chocblock.
 Can't say I've ever seen a 38mm dual box, mind, though they probably
do exist! You won't need one, though, under this proposal. Downside is
you still have to do a little expansion of the existing mounting hole -
instead of making it 12mm deeper, which ranges from a PITA in brick to a
total non-issue for wing-mounted plasterboard boxes, you need to extend
sideways by a bit under 1cm, since two singles are wider than one double.
HTH - Stefek