Mineral Insulated Copper Covered Cable (MICC) sometimes called Pyro one of the best cables invented lasts forever except if Damaged the insulation is a powder called Magnesium Oxide I believe, but it is highly absorbent and takes in moisture very easily which is why it has a special termination a brass pot filled with Compound to stop water ingress. unfortunately you cannot repair it without cutting back at least 300mm to 450mm then you could probably re use the existing Gland,Pot, etc with new compound, you would need an experienced spark who still has all the right tools to strip the outer sheathing a Ringing tool, a Pot Wrench and a Crimper I have all the tools and the know how but it may not be your cheapest option, I have heard of someone chasing the damp back out of the cable with heat but it is unlikely to work and you could not reseal a damaged section you may have to totally replace the section of cable, with a new PVC/PVC cable.The plasterer should have third Party liability insurance and you should ask him to foot the costs completely it is his fault not yours. as an electrical contractor we have £5,000,000 of insurance cover so if we
damaged your plaster we would be insured and you would be covered. all the MICC parts are still obtainable and all the sizes are marked on the side of the pot and Gland so its easy.
I hope this helps if it does hit the like button
Thanks
Kelvin