Whats the alternative to adding a spur if the ring main hasn't enough slack
to reach the new socket? Could I break the ring with two junction boxes
and add two wires to extend the ring to the new socket, if thats ok what
about a double wired spur?

I don't really understand the theory behind this spur thing, is it the
rating of the wire from the ring to the spur or is it just that if you have
too many spurs they you'll probably have too many sockets for one ring?

Can somebody please clarify this for me?

Many thanks
Roger