There is a which independent report on what you should pay electricians to do an array of jobs... search google 'How much should i pay an electrician'
It wont allow me to post a web page address here.... Stupid I know...
It states in here an average of £100 - However it is a few years old now. I'd say £120.00. Depending on the 'job' how/where you take a spur from. But this is a good guide...
Hope this helps.
including the waterproof socket approx £100-120 as it will require a minor works certificate as it is a modification to an existing circuit.
You will therefore require a PART P electrician holding a relevant competent person scheme membership.