A little more info:

I've already run the ring with 4 sockets in the conservatory, and have
the ends of the new ring run to the back of the existing socket (not
connected yet).

The metal back box is a deep one (about 50 mm) so there is easily room
to fit the choccy block behind the socket.

The choccy block I have is rated at 30A.

The choccy block will be accessible by unscrewing the socket.

I don't have spare cable in the existing socket to put a junction box
on.

I don't have access to a crimper & crimps (without buying one!)

Given these circumstances does anyone see any problems with using the
choccy block?

Thanks Paul.