Dedza pottery (Paragon Ceramics Ltd.) is probably better known for its arty products. However, they also produce a range of industrial products, including refractory bricks (which have been shown to withstand temperatures well in excess of 1000°C. These bricks are of exceptional quality, extremely heat resistant and certified to different industrial standards. Given a sample they will make an identical replacement brick for a furnace, boiler or other industrial appliance. They also make bobbin insulators for low voltage overhead power lines.
Don't be fooled by their retro web site. Here is the Dedza Pottery website. The owner Chris Stevens has a trough of knowledge about ceramics and their characteristics. It is a real pleasure listening to him explaining the interactions of different materials during the burning process. These days he spends most of his in the sun on Likoma Island working on his retirement project.