Thanks for your info sheet Rob. However, in the best traditions of the
stubborn Interwebs, I pressed on (rim shot) with the materials originally mentioned.
Here's the knackered part. It's not a frame as originally mentioned it's the bottom part of the handlebars for a Pino. It does, however, have a head tube type thing. Note the rust hole, generally considered to be a Bad Thing.
An offcut of 16mm 316 stainless in a funky rolled thread with nuts to match plus a brace of domed washers. I mounted this vertically in the Workmate by poking the bar through on of the many holes. Slid adjustable spanner over the lower nut and dropped a suitable pin into another of the Workmate holes to act as a stop for the spanner, thus leaving me two hands to wind the upper spanner. I applied a smidgeon of grease to the outer edges of the cups and did a gentle tappity tap to get them started off square.
Hideously expensive replacement bars with bearing cups from the old set.