If you threw this question into a room full of electricians, you're likely to come up with fourteen different answers.
It wouldn't really be a spur... just an extension to a radial circuit. (You'll also get a whole bunch of arguments from electricians over that statement as well.)
Anyway, you're proposed set-up would comply providing the breaker is no more than 20A
Mmmm, the great, grey area of Certification and Notification.
Officially, you're not installing a new circuit.. so that's Ok. Also, you're not actually in a kitchen or bathroom .......
But, also officially, you aren't simply replacing a socket... you're introducing new ones, so it would be notifiable. That does mean that, by law, it should be done by a registered electrician.