To answer my own question, I found this place on eBay: http://www.ebay.com/...984.m1497.l2649
Got two (one's a present) 350mm square 12mm thick steel plates. They were pretty coated with a grubby rusty substance when they arrived but a 24 hour soak in normal vinegar got that off with no effort at all. A good wash in soapy water and an hour or so seasoning in the oven with a thin layer of oil and they were ready to go.
Made my first batch of pizza yesterday and will be posting more about it when I get the pictures sorted, suffice to say they were the best I've ever made. Couldn't be happier with the steel. Especially as each one cost just over £30 delivered, compared to over $200 for the equivalent from bakingsteel.com.
Edited by &roid, 21 September 2013 - 04:40 AM.