I tried a new idea on some old chisels today and it worked a treat.
I got a local engineer to machine the top of a sheet of steel flat for me on his shaper machine. I now have a 1"x7"x11" lump of steel that is dead flat. Placed 600 grit wet and dry on it and in very short time had chisel back nice and flat.
Less than £10 for machining and steel was free scrap. I now have a'surface plate' to check everything on and flatten blades etc. to my hearts content.
Unfortunately I have vowed to only buy LN from now on and so the surface plate will not see the sole of a plane 8)
I highly recommend this approach - check your yellow pages for machinests
Tony