Is it legal to add extra ring main sockets yourself or does it need an electrian to do it
It's not illegal to perform this task.
But you would need to follow the guidance in BS7671,
I would
*Check the existing circuit is suitable for additional sockets
*Investigate on the existance of 30mA RCD protection on the circuit being added to, if not then that protection should be made to at least the newly buried cable and new sockets
*Route cable in permitted safe zones
*If possible extend the circuit rather than spur
*If more than one socket outlet (single or double gang) is to be added at ring final socket, then a 13A FCU should be fitted at the intersection between feed socket and first added socket.
*Check cable sizing is correct for circuit
*Be aware that standard junction boxes or joints in cable must be easily accessible for maintenance, inspection and testing.
*If joints are hidden and inaccessible, then joints that are considered maintenance free must be used.
*On installation, a number of test should be made to confirm the safety of the circuit and the operation of safety devices complies to BS7671.
*Then this information should be documented on either an electrical installation certificate or a minor works.