The great gardener, and sadly departed Geoffrey Smith, advocated waiting until the tubers begin to sprout, take a sharp knife with a steralised blade and divide the tubers. Dust the cut with sulphur or tobacco ash and leave the cut to dry.

I did that with a BIG Begonia tuber I had - I called it "Big Daddy" -h and it was never the same. The cut up pieces all grew but didn't flower as spectacularly and I regretted splitting it.

I had a really big tuber for many years and it flowered lustily. This last season I went to pick it out of the net and my fingers sank into gunge. I chopped off the good bit and popped it in a pot, as it was, and it bloomed. It's back in a net hoping for no vicious cold spells.

thanks for the replies