A ring main as the name implies, is a wiring ring between a circuit breaker on a consumer unit and the sockets it supplies.
One wire leaves the consumer unit and connects to the first socket, a second cable then leaves this socket and goes to the next, at the last socket in the chain, the last connection goes back to the consumer unit creating a ring. The power load is shared both ways around the ring allowing use of a smaller diameter cable than a radial wiring system. This smaller diameter was one of the principle reasons for its introduction after the war in order to reduce the amount of copper which was in fairly short supply.
Older wiring systems have remained in use until fairly recently, my grandparents house was only rewired in 1987 although many were converted earlier.
The wiring scheme has some criticisms including less apparent fault symptoms and more lengthy testing requirements which aside from the complexity of retroactively adopting a new standard have probably contributed to its limited use outside the UK.
The Wikipedia article contains some more details on the standard, its history and issues but adequate training should be arranged before making any modifications to existing circuits.
http://en.m.wikipedia.org/wiki/Ring_circuit
It's worth noting as well before considering any work that Part P of the Building Regulations may well apply, the IET has a good faq on the subject which is worth reading:
http://www.theiet.org/publishing/wiring-regulations/part-p/
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.
Best Answer
Yes, but a few points need to be observed to make sure it complies with BS7671 IET 17th Edition.
Firstly, the terminals must be installed in a Wagobox (when breaking into existing ring wiring). This needs to be UK sourced to ensure you get ones that are stamped with [MF], meaning that it's classed as BS 5733 Maintenance Free box.
To comply with the IET regulations, each Wagobox is limited to 50Amps in total. The compatible terminals, 773(solid) and 222 (lever), are also derated to 20Amps.
Therefore, for a regular 32Amp ring main, you will need to use two Wagoboxes - one at the start and one at the end of the extension. As the load from any socket is shared in both directions of the ring (albeit unevenly), the derated 20Amp terminals are sufficient.
Before:
After: