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Surface mounted wire and socket it should cost around £100 but good luck finding someone who is prepared to do less than 1/2 a days work. You'll probably end up with quotes around £250. If you want it chased and plastered with a flush socket then £250 would be reasonable.
You could do it yourself integrated it into an existing circuit. It's only the addition of the circuit which requires certification.
How much do you reckon it would cost to have a spark fit a new socket within a couple of feet of my consumer unit? I want to be able to plug shit in under the stairs where the unit is.