ok, to DIY safely you will need...
a copy of the regs and a copy of the on site quide.. ( about £100 for both )
<etc>
A copy of the regs
I'd say that to extend the ring he needs
some 2.5 cable
some back boxes
some sockets
some grommets
some plugs and screws
something to cut the boxes out with
a bit of common sense
perhaps one of those plug in socket testers
earth sleeve
cable sheathing/channel
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?
BUt in reality, what he probably will have is:-
some cable - might be 2.5, but whatever comes to hand will do.
some back boxes
some sockets - prob second hand, been in the garage for years.
some grommets - haha, unlikely
some plugs and screws - or a tube of grip fill, oh sod it, this caulk will do
some thing to cut boxes out with - knife and fork, but not from the best set, Mrs. will go nuts!
A bit of common sense - or some random info from a web site
perhaps one of those socket testers - probably not bother with that, eh?
earth sleeve - don't worry about it, its only the earth
cable sheathing/channel - ooohh, a sheath and some chanel (No.5?) sounds like the start of a good night
screwdrivers and pliers - jewellers screwdriver, bread knife
Consider, if the OP was doing this with a disregards for regulation, i would not be so bold as to guess it was safe.