The mandrel arrives, an ebay purchase from jewellers-tools, one of few items I have bought from them. Good value for money and fast delivery.
Its nice and heavy and does the job. The ring is now round, and I spend ages polishing it until I am happy with it. The join is not invisible but it is good enough for me. I think it may be a little small after all, but will wait until he tries it on before I do any more to it.