Kent,
This has happened to me. It depends on which Cereus it is, but usually I remove the top, set aside to callus and cure (all winter if no greenhouse, keep it dry) and root in the spring. You can then put it with the parent plant as a low stock branch (looks like it is growing from the base of the plant). The stock plant then will usually put up branches. Hope this helps.
Dan
Lincoln, NE USDA 5

Thanks Dan.

I posted an image as a new posting as I couldn't work out how to put an image on a follow up posting. Even then I hoped to post two shots so that you could amybe ID it but it would only upload one despite small file size. Anyway, the subject heading is broken Cacti.

Thanks again for your prompt reply.

Kent, Melb, Australia.