I am waiting for my new drill to arrive so have had to work on the outer shape before removing the inside plug. There will be a sizable lump to cut off each corner which is worth keeping. I have isolated each lump today so that I can drill the holes to split off each one when the drill arrives. I also had time to begin levelling off the flat face of the marble. There was a split that was almost horizontal but still diving towards the heart of the marble. This was making me a little anxious as I did not know how far into the marble the split would travel. On chopping away at it however, I see that it runs out quickly and will not cause any problem.. Once the marble on this side is the same thickness as that of the other side the split has gone. Phew!