As you say, there is nothing electrically wrong with having 2 (or more) sockets on a 4mm² unfused spur. There is, of course, an argument that one has to avoid having too much load attached to one point (fairly close to one end of the ring) on a 2.5 mm² ring - but the people who question whether it is acceptable to have two double sockets on the same (4mm²) unfused spur will usually have no problem with two double sockets being separately spurred from two points just a few inches apart on the ring.
No, for the 15th onwards it's been one single or one double.
As I've just written, you are, of course (at least for 17th & 18th - I can't speak for earlier), talking of the guidance in Appendix 15 of BS7671. To comply with the the actual regulations you only have to satisfy yourself that it is "not likely that any cable will be overloaded for a long period of time".
I find similar, but there is uncertainty as to the 'installation method' and environmental conditions to which the quoted CCCs relate.
It does - so goodness knows what the truth is?!
I'm not sure that people who need someone else to read the regulations out loud to them should be electricians.
The conductors of a stranded cable have a much higher surface area than a solid core
Surely that table is as much the truth as any tables in BS 7671?
Not all of the 'surface' area is on the surface and there are many air gaps.
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