The neatest way IMHO, is to change the sunken socket to surface mount(25mm box) and then use one of the provided knock-outs in the box for the new cable to exit.
To do this you need to know whether the wires are long enough first. You drill a hole in the back of the box for the wires to come through. The little brass threaded inserts where the screws go can be knocked(or drilled) out the back of the box. This allows you to use very long faceplate screws to go straight through the double socket, through the plastic box and screw into the existing metal back box in the wall.
Normally you just knock a hole in the plaster to take the cable into the back box.
bit of filler afterwards if required
or
you can get spacers to space the socket off the wall a bit. And file a bit out for a cable.
Not really what they are designed for, but neater, and less work.
Thank you both for your replies, I appreciate it. The wall is tiled (the fused spur is for an under cabinet light, the double socket is just underneath so easy location to take a feed from) so anything sunken is not an option really.
Both ideas sound good, neat trick with the extra long screws passing through a new back box.
Might look at the spacer option first, didn't realise you could buy such a thing. I did consider filing my own 'knock out' in the mains socket but didn't know if this was an acceptable practice or not.
Edit: Just looked at spacers online and some actually have a knock out for a cable so that would solve it.
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