Hi Charlie - much clearer...
Firstly a power circuit should be a lot more than 5 Amps - In the UK 5A is for lighting circuits and 15A for power. You may well have the wrong fuse holder driving the power sockets and if so, even though it's best to use the correct fuse holder, it will need a 15A wire in it.
The problem may have been caused originally by a bad connection in the extension lead made worse while you changed things around plugging & unplugging - put the extension to one side and just concentrate on the socket. Use a plug-in lamp or small appliance to test.
Fit 15A fuse wire in the holder at the distribution board
If the socket still doesn't work and the 15A fuse wire remains good then there's a broken connection to the socket (could be Live or could be neutral) - if the socket has two sets of wires; it probably will have two sets - each of them will be routed via other sockets back to the distribution board to form a ring and there could be broken connection at the back of a socket on each side!
Take it a step at a time. Fuse first at 15A - then the other sockets on the same circuit - then the suspect socket. When all that's OK look to the extension lead that seems to have started the whole thing off.
Good luck...
Peccavi
August 2009