The word spur is a problem as it's meaning is not really defined. Although it says
Spur. A branch from a ring or radial final circuit.
a branch from a radial could also be classed as a radial so reading alone will not help.
So what is required is defined with 433.2.2 and that says where the protective device is at the termination of the cable rather than at origin then max lenght is 3 meters. Also has neither branch circuits nor outlets between origin and protective device.
So there are two spurs, the fused spur where the protective device is at origin and the non fused spur where the protective device is at termination i.e. the fuse in the plug.
With the latter clearly you can't have more than one outlet. So each spur will have a single FCU or socket outlet connected to it.
Because drawing more than 13A from any point on a ring could cause one leg to be overloaded one can't take two spurs from same place on the ring. However since you could have two sockets right next to each other on the ring and a spur from each it does not really make too much sense other than the problems in ensuring good connections are made.
Using a loop impedance meter one can of course test the connection and also the logistics of if another socket could be added and still ensure the protective device will operate correctly and volt drop is within limits. However most DIY people don't have test meters and connect wires and trust to the Lord. So with no testing one does have to be very careful working in any confined space is not recommended.
Today all new sockets need RCD protection and many older houses don't have RCD's in the consumer unit. So in most cases a RCD FCU is the only way to add a socket. Well one could use Ali-tube cable and a RCD socket but buying short lengths of Ali-tube seems to be a problem so in real terms the RCD FCU is only option.
With radial circuits much depends on existing cable size, new cable size, and fuse size to if a FCU is required. I wish a spur was defined as "A branch from a ring or radial final circuit where the overload protection is at termination" but it's not so a radial and a spur can be the same thing. So the cable after the FCU could be called a radial or a spur which means easy to make mistakes when reading regulations.
