I've finally got my arse in gear and phoned a couple of local steel stockholders after a bit of mild steel 300mm x 400mm x 6mm. First one had nothing, the second had 8mm, so I went with that. 5 minutes later the person doing the cutting phoned to say they had no 8mm and would 10mm do? A short think and I said yes.
It's Quite Heavy. I've cleaned it (hot water and metil scourer) and it's now seasoning (Hot oven, veg oil, repeat a couple of times. Allow to cool.)
We shall see.