The area to flatten is 900 x 400mm and my surface plate is 600x600.

Yeah I talked myself out of it a few times. I'm not sure its going to be very economical, the BT30 spindle itself is a substantial cost.

Chaz, I found your concrete build very interesting and I've thought about a moglice/DWH approach. I'm lacking a big enough reference surface though, any ideas on how I could overcome this?

I'm starting to wonder if I will have to leave the base as it is. If I could be confident in the way I was measuring it and I was sure it was flat to say 0.02-0.05mm I would probably be happy and just shim/scrape it later. Does anyone a good way to measure how flat a surface is that's bigger than your reference surface?