I have one double socket in a bedroom but would like to add about 4 new double sockets. I am placing them into the wall and under floor boards
I assume it is the cable that is being routed underfloor rather than socket outlets?
but can anyone tell me if I have to use a loop system? Or can I just used a junction box of the main socket to serve all the other sockets
I would steer away from using junction boxes and the standard junction box is no longer allowed in areas that can not be accessed for maintenance, inspection and testing purposes.
The way you extend the circuit would depend on the type of circuit you have.
Ring final circuit would need extending in a way that keeps the loop/ring continuous and loaded evenly and if you dio spur sockets, you can only spur one outlet from any one socket on the true ring. The radial is a little more flexible and allows for extending spurs of multiple sockets.
You can also use a fuse connection unit rated at 13A, from any one socket on the true ring and then add as many spurred sockets as you like from that fused spur, but this will only allow for 13A total load on the additional sockets.
Then you must consider the addition of 30mA RCD protection for any newly buried cable and all new socket outlets, it maybe that this protection already exists on your system and this circuit, but would need confirming.
You must also route cables within the permitted safe ones and use the desired techniques that comply to building regs when chasing cables and holing joists.
There is plenty of information in Wiki in Electrics section.
//www.diynot.com/wiki/Electrics:index
One of the most important things that is very often overlooked by the DIYer, is that the circuit and any addition to it will require proving it is safe before it is re-commissioned, this will require using the correct inspection and testing procedures and calibrated testing equipment. This would also require to be documented in the form of a certificate.