Electric to my shed

Ideally your shed circuit will be seperate to your house circuits incase of any faults outside effecting your house circuits. However you will be fine spurring of a socket depending on the loading requirements on the circuit and what is to be added on top. There are a couple of ways to go with that approach. Ultimately though any circuit added or added on to needs to be safe on completion, the circuit cannot contain rewirable fuse wire as overload protection, RCD protection required and any services entering the property will need to have a 10mm csa earth cable to within 2.5 meters of there stop tap and labeled up. Once you have checked these you will be safe to contact an electrician to design an installation for you and get him to install it, Only use registered and qualified electricians. Regards Ben Evans

Ultimately the issue will be one of how much load will the new addition draw? I would strongly advise against feeding from the lighting circuit. You could feed from an existing socket, As a precaution i would use a switched fcu to then add the required number of sockets or lighting points. this will allow you to extend from an existing ring with more than one point.