how to fit an external light from a socket inside....

Requires connecting to your rcd circuit some where whether it is connect to existing circuit or to a spare switch on the rcd box just for safety reasons

Hi First, this is outside work so it is notifiable. Second, and teh most important thing is how do you intend to test based on the earthing system that is in place at your home? So if you do this DIY without notification and get caught, you will have a criminal record and a hefty fine to pay, second, getting the circuit to work is the easy part, testing and understanding the test results to ensure a safe circuit is where the training and technological nous comes in. Ask for a Part P registered electrician to do the work for you, you will then be covered and safe.

Firstly, outside work is no longer notifiable to Building Control. Secondly, do not just blindly employ somebody who is Part P registered without getting references or having knowledge of their actual qualifications. The scary reason for this, is that a lot of so called "electricians" who are Part P registered, have undertaken a 5 week course and paid some money to a scheme which has then classed them as "competent" which is impossible after such limited training. However, it's not recommended to undertake this work yourself, and to employ an electrician because it needs to be done properly and safely, and tested to ensure exactly that. A fused spur, preferably with a 1 or 3a fuse, would be suitable, but the cable run, and cable used also needs to be suitable. It's not really a DIY job.