Trevor Smith wrote:

I thought that all spurs on a ring main are fed from a 30A fuse/32A MCB
or have I got it wrong all these years

No, you're quite correct. The 30 or 32 A device only provides s/c
protection for the cable; overload protection is provided by the 13 A
plug or FCU fuse(s) downstream. That's why there's a restriction to one
load point (it being assumed that the load on a double 13 A socket won't
exceed 20 A, and that only one side might be overloaded at any
particular time).

--
Andy