If I had one :)

Well put a 1.5thou (.02mm) feeler gauge under it and I can push it through in places, so I suppose the answer is 'not very flat'

I have 2 pieces roughly the same size. I was wondering about lapping them together to get it flatter. Thoughts anyone?

A 12" sq 3" thick granite surface plate is about £35-£40 new. So it might be worth the effort.