Electric connector

That is most definitely not right, as is, it will let water into the plug. Is there a rubber seal missing or is it just inside the plug where the cable enters. Rather than shortening the wires, it might be just a case of ensuring the cable is pushed into the rubber seal (cable gland) thereyby making a water tight joint. Undo it and check the seal is in the correct place over the cable outer sheathing and the action or screwing it back together again should clamp the seal tight.

peedee

No sign of a sealing gland anywhere and the cable won't really push further in, hence my wondering aout shortening the wires.  I'll have to see what I can find in the miscellaneous bits box.

Sounds like the gland is missing which maybe why it was wrapped in cloth tape. You could wrap it up again with tape or buy an new plug because I do not think you will be able to buy a single gland. A new plug is £8.49 on Amazon

peedee

I would replace the plug just in case to ensure a watertight connection.  It is a simple DIY job but recommend that you cut the cable caravan side just outside the old plug (or leave sufficient connected to the screws in the plug)  so that you can see how to wire the new plug; simple but a little fiddly.  Been there done it!

Things may be a bit more serious since what you show in the photograph is exactly what one sees when the cable has been put under tension and the cores have stretched more than the outer sheath. 

The plug has probably been fitted without a gland because of the size of a 12n and 12s cable both through the entry point.

It is possible to buy a plug designed to accommodate two cables such as This

But it will probably still be a struggle to fit and you may prefer to replace the two cables with a 13 core and a normal plug.

There should be a rubber gland in the screw section that tightens onto the cable sleeve and stops the cable being pulled out, looking at your photo just in front of your hand there looks to be a witness mark of when the gland was tight on the cable, I suggest you dismantle the plug and make sure the rubber gland is there, the wiring is still connected and if OK reassemble with the gland over the witness mark.

Looks like a 13 core cable to me.

 You can just make out were the gland came to on the outer cable insulation, as suggested replace the plug but take photos of the wiring before removing the old one, you may have not left enough slack in the cable for turning 

Sorry if my response was a red herring.  In the small pic on my phone screen it looked like a grey cable and a black one side by side.🫢