ok, to DIY safely you will need...
a copy of the regs and a copy of the on site quide.. ( about £100 for both )
A copy of the regs
I'd say that to extend the ring he needs
some 2.5 cable
some back boxes
some plugs and screws
something to cut the boxes out with
a bit of common sense
perhaps one of those plug in socket testers
screwdrivers and pliers
Consider, if the OP's current installation is safe, and he just copies it, then the new work can not be unsafe , can it?