Shopping in Aldi's his afternoon and happened upon black or grey granite 'Worktop Protectors', to allow placement of hot pans on vulnerable work surfaces. There are two deals available, both priced £6.99. One (which I purchased) is a single granite slab measuring 400mm x 300mm, the other comprises two slabs, each 300mm x 200mm. The slabs are about 15mm thick so are substantial, but the most valuable aspect, from a non-culinary point of view, is that they are ground to optical flatness and so make jolly inexpensive surface tables for marking out, checking and measuring on. I checked the flatness by looking at a reflected fluorescent tube whilst tilting my slab this way and that, without detecting any ripple or distortion. I reckon that means the surface is adequate for most folks need, SO .. if you're lacking such an item in your workshop go and bag one quick; stocks are limited.