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You can only spur once. If you extend the ring you can then spur from that later if you want.
It's preferred that you make connections in existing backboxes rather than just splicing into the ring under floors or in the loft etc. It's just a question of convenience in the future if someone is working on the circuit or fault finding. You have to balance that with convenience now of course!
Airhead-
Thanks, think I will go for extending if possible then. Don't think I was clear in my description... I wasn't suggesting a string of spurs all off the same point, it would have been several spurs so the cable would drop straight down to each socket. But still I think extending makes more sense like you say. I will have to get back up in my horrible loft and see what I can find, there's cables everywhere!
Arducius
I want to add some sockets to a ring main, is it better to spur off or extend the ring?
I have reasonable access to the wiring in the loft and I can drop new cables down inside the wall to back new boxes or - probably my preference - through the ceiling (and run cable through surface mounted conduit) from there to pattress boxes.
Spurring off would use slightly less cable and fewer connections in total but is there anything else to consider (apart from total potential load on the circuit)?