Adding an extra socket - possible to add another junction box to the same length of wire on the ring main?

Using table H2.1 Type of circuit A1 ring 30/32A 2.5mmT+E Using regulation H2.4 spurs In an A1 ring final circuit a non fused spur should feed only one single or one twin/multiple socket outlet or one item of permanently connected equipment. Also such a spur should be connected to the circuit at the terminals of a socket outlet or junction box or at the origin of the circuit in the DB. So the answer to the original question is yes (for one extra single/multiple socket outlet) as per regs also laid out by ESS above. Note: If you wanted to run multiple sockets of the ring you could use 4mmT+E so you would have a radial of the ring.

As you'll be doing DIY alterations to the wiring, you need to register your intent with your local building control department and pay their fee (typically around £200-£300) to get part P building regulation consent. Once you have consent in place you can do the work and on completion an inspector will call round to check and test its ok and issue a part P certificate. As a general rule the number of spurs from a ring circuit is unlimited. However you will need to hire a continuity tester and carry out a check on the ring circuit to verify that it is wired as a ring first, as its common to find the ring has been physically broken in the past due to poor quality additions. If the circuit is not already RCD protected then it will need an RCD fitted as all new additional sockets fitted must be RCD protected in order to comply with BS7671:2008.

You can add as many spurs as you like as long as you protect the cables accordingly. You can not leave a 32A MCB/Fuse in place and use a 2.5mm cable to spur off the ring. A single 2.5mm cable needs protecting at a lower rating than a complete 2.5mm cabled ring. Many a muppet do not know this. The risk of overload may be low if you spur to a single/double socket but the installer never knows what comes after so he must install his work to current regulations. Also, note that junction boxes, if not done correctly, are a fire hazard. Depending on where you are doing this addition you may be breaking the law doing it yourself without notifying building control. All in all it really is easier and cheaper to get a registered electrician in to give you some advice