You can do that, aslong as the existing back box of the socket is large enough!!

the easy thing to do would be to remove the spur cable, put a new one in to supply a fused connection unit then you can loop asmany spurs as you like off of the fused connection unit as you have the fuse to protect the cable.
if im adding sockets, given the choice i add em onto the ring, only exeption is gas hob ignition and the like.
this is the way most sparks will do it

ss

blimey...sleveing earths have been a reg since the use of rubber...
its not old indian rubber is it??
(black sheath)

ss

Hey Owen, sounds like you did the right thing, if you ever come across earths like that, best to carefully un-twist and sleeve seperatly, it can cause many problems when done together.

A few issues are firstly it actualy stretches the copper and therefor weakens it and this also affects it's electrical integrity, earthing conductors can then break when detatched or untwisted, this depending how much it has been twisted. it also goes on to make it difficult for fault finding and testing purposes.

If found it should be sleeved seperatly as intended, however it's not the most dangerous thing that can be found, however it is considered bad practise now and sould be totally avoided.