Have used John Parry-Jones, based up at Taffs Well, for a good few jobs- very much a craftsman ("proper" engineering!!) and very impressive work, together with being reasonably priced: will dig out his details and post them.
Try Food Quality Engineering, Chris or Graham, 02920 553002. They are a father and son business mostly making one off pieces for the food industry but the y have a steady trickle of boat work coming in. They are next to Fairwater police station and opposite me!
Give Peter Richards a call via Dickies in Swansea. He works from Dickies workshop and is a whizz at SS fabrication. He has just made up some chainplates for me. I've seen some hand rails he has made for powerboats and they are top notch.