Logically I can use 2, 3 terminal junction boxes, or a single 6 terminal junction box to extend a ring main (inserting a loop to the new socket(s) I am installing). However are there any practical issues with a 6 terminal junction box – e.g. space for the cables?
What would anyone inspecting the wiring expect to see?
(Note ring mains are not normal outside of the UK)
I don't know about the inspection issue, but personally I'd be wary about using a single 6 terminal block to extend the ring.
I assume you are breaking into the ring at a single location and inserting a loop to the new socket(s) you are installing. In that case I'd use two 3 terminal boxes at each end. Depending on the length of cable and number of sockets I might be tempted to install spurs, but that is just me.
However, I would suggest that you consult an electrician to see what the approved method is. The last thing you want is to have to redo the wiring when you come to sell the house. The easiest way is to get a quote and have the electrician explain what he's proposing to do.
Electrically I see nothing wrong with it assuming a 20A junction box (same rating as is required for the conductors on a ring circuit).
Wiring will be quite cramped, you will likely need to think about which cable entries you use and carefully cut and bend the connectors to get it all to fit.
Remember that screw terminal connections must remain accessible for inspection. If the joints won't be accessible for inspection you will need to use a suitable "maintenance free" junction box (e.g. the Ashley J803). I haven't seen such boxes in 6-terminal so you would need to use two seperate boxes.
