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IANAE but that's how I would do it. Obviously you'd have to "open up" the ring. And, you can just get longer screws if the faceplate sits too far forward after plastering. Annoyingly they are only available in boxes of a million (you might get smaller sets off ebay), but they are very easy to come by.
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That should work. Best practice you make any joints inside back boxes so you don't need to worry about maintenance free joints etc.
There's loads of techniques that go into a small job like this and really you could be testing the circuit afterwards to make sure everything is still good but I'm sure a lot of this kind of work gets done without any testing or paperwork.
I guess you have an RCD on the circuit already in which case you can bury the wires in the wall.
You're probably right. I've got my head round adding one socket, but I guess two is like this (hopefully the illustration makes sense)
How does it work in terms putting recessed boxes in before plasting? It's a skim coat so I assume I just recess to the current wall depth and any increase is dealt with by the screws?