I start them off in the propagator, then when you can see where the new shoots are, cut them with a knife so that there is at least one shoot on each piece. I dust the cut edges with sulphur powder, leave them overnight to dry the edges a bit, then pot them up as usual. I was given this way of doing it by an elderly gentleman on the Begonia Stand at the Gardeners World Show in Birmingham many years ago and its always been successful.

Thank you Jaykaty for your help, I will give this a try.