The initial idea was to make a really heavy table that wound't shift about but at a quarter of a ton just for the top (so says the steel supplier) I think 20MM might be a bit much.
I was talking to Dad about it as it'll be sat in his work shop and we're thinking somewhere in the region of 12MM with added bracing should do it. We're also thinking about getting holes laser cut or drilled into it so it can be used a bit of jigging table type thing.