shagster said:I mean the current that flows both ways round the ring, from the source meeting at the appliance socket can presumably be double what it would be on a spur.
That's only true if the socket is at the mid-point of the ring. In practice the current will not split equally between the two cables so, while a 2.5mm 'spur' direct from the CU (better known as a radial) is typically rated at 20 amps, a ring of the same cable is only good for 32 amps - and you could still overload it by putting the full load close to one end.
and also said:It's got two routes, therefore twice the area of copper --
Since you're obviously interested in the subject, you should know that doubling the area of copper will not give you twice the capacity if the doubled up copper is all in a bigger cable. I'll let you think about that one; it's a thermal problem.
and also said:-- or it joins to the CU in two places.
A ring starts and ends at the fuse/breaker in the CU. Having said that, I inherited an upstairs ring which started and ended at a junction box under the floor. This in turn was on the end of a 4mm radial from the CU and, just for good measure, that radial stopped off at a single socket on its way to the junction box. I can only imagine that whoever put it in had a lump of 4mm cable going spare!