No worries, Ern.
On a serious note, the fence idea will work, but only if the handle is a circle now. Otherwise you will just saw a perfect reflection of the current state of the surface (although you may smooth out some of the irregularities).
I was wondering whether you could make a kind of holder to put in a 3jaw chuck. What I had in mind was a rod with a flat plate welded trasversely at one end. The bevel end of the chisel would be laid on the plate and clamped to it by a plate held to it with screws. The rod would go in the chuck and the tail would hold the handle centred. I'll do a drawing and see if I can post it.
If it were done when 'tis done, then 'twere well it were done quickly