Replacing the sill on an X type is relatively simple, but you do need to be able to weld. I had never welded before, but I decided to replace the sills myself. Welding machine £80 from Aldi, Self dimming welding helmet £20 from Aldi, new sills that were formed to fit my car £80 for the pair, welding rods £10 from ebay, also purchased a couple of flapper grinding discs £10 .
Watched a few videos on Utube, then set about welding a couple of scrap pieces of metal, took five attempts to weld, then suddenly I got the hang of it, cut out all the rotted metal, then fitted the new sills, I used scissor jacks to position each sill, then I drilled a few holes and put screws in along the top of the new sill and into the old sill then started welding. I had before used my angle grinder with the flapper discs to clean up the old sills that I had left.