I was talking to a very nice man from
Special Plasters and their ceramic blanket is the cheapest I have found so far. He had never heard of fireclay though and went on to name loads of other stuff I could use but I had no idea what he was talking about. He said that he had some crushed firebrick lefteover from a job. In the end all he had was about 10kg of it, but he said I could have it for the cost of delivery. It will be here tomorrow.