Wow! This book must have been fairly popular at one time.I have an old Home Electrics book of my Dad's that I used to read all the time when I was younger and fascinated by electrics. I distinctly recall it stating that you could take an unfused spur to 2 x single sockets or 1 x double socket, I've just dug it out and attached some pics. No regs are stated but thought some of you might find the history behind the varying 'can you spur from a spur' opinions interesting.
I still have a copy as well as did a bloke who lived about 1-2 miles away from me (his son, who now lives in the house, probably still has it).
I think it dates from 1984 (well my copy any way, if I remember rightly when I last looked in it).
It even has a section on replacing your Consumer Unit, including arranging for the Electricity Board to remove the mains fuse, then fully detailing the steps to be taken etc and upon completion for the work, arranging for the supply to be re-connected.
Of course this was decades before part p and as far as RCD protection was concerned, this at the time would only have been required for TT systems (just one ELCB between the meter and CU).
It also has a section on wiring a converted loft, again in a lot of detail.
Regarding 30mA RCDs, they are referred to as a 'High Sensitivity Current Operated ELCB' (although they were probably quite unheard of at the time) with it stating 'There is now a Consumer Unit available where certain circuits are protected by a High Sensitivity Current Operated ELCB, where as other circuits such as indoor lighting are controlled by a normal mains switch', although the actual term not used, it is basically referring to 'Split-Load Consumer Units'.
Although many regulations have changed/come into force since it's publication, I must say, at the time, it would have been a BRILLIANT book.
Regards