In reply to Philip:
Hi Phil
To cut to the accuracy you stated you will most likely need a cnc type machine.
If you can settle for more rough and ready cutting I would use a tile cutter. This should give you more control by offering up the piece to the blade on a base instead of using a hand tool.
I have cut small ceramic tiles, 4 x 10 x 50 mm long using a tile cutter v well, just not to +/- 0.1 mm.
HTH Davey