After a half-dozen visits to so called "plumber supplies", I was told that these waste fittings are supplied with the sink, and not something generic ... which is why none of the generic fittings looked anything close.
Unfortunately I have no idea who made the sink.
I have managed to use a soldering iron and clamp to repair the piece for now.
Can anyone suggest a source for these manufacturer-supplied wastes ... I thought the sink was astracast, but looking at their website, it's close, but no cigar.
Of course I may have fallen foul of what the chap who told me the sorry tale warned me could have happened ....
"You need to contact the sink manufacturer ... that is, if they haven't changed the design".
RANT I thought one of the spurs to the modern age, was the introduction of mass-production, and standard, interchangeable parts. Why hasn't anyone told the plumbing industry. /RANT