my best mate is a part p sparky and has just rewired my WHOLE house and in doing so has told be everything and i mean everything about what is legal etc

from my understanding an average joe can touch/change existing electrics which includes extra sockets as long as this doesn't excede the ring main output

to answer your question, i think that you can replace a blown amp in an existing fused spur but not to fit a new spur as you'll have to be qualified and that addition requires a part p cert