this work will have to be done by an electrician registered for Part p of the building regs, although heathfield is correct in one respect,you can have as many spurs as you like on a ring main, you cannot spur from a spur,if you do not want to call in a sparky,the only way round this is to run an extension lead with a plug top on,this can then plug into the bedroom socket, with a trailing socket in the shed, this should go through an rcd adaptor to eliminate the risk of electrocution, if it is to be hard wired, it will have to have an rcd protecting the circuit.