Today, after phoning every single plumbers merchants for 30 miles, including Astracast suppliers, it seems I may as well have been asking for a return ticket to the moon. You can't get them. They're not sold separately, and the one that Astracast *do* sell won't fit a sink older than 3 years.
So, for the want of a waste, the sink was lost. You can bet I'm not buying an Astracast again. Bog standard stainless steel for me - whatever the Mrs says.
Unfortunately I inherited the sink, although even if I had bought it, I would never have imagined it needing scrapping because the waste can't be supplied.
On a wider note, have you ever refused to buy anything, when the shop can't supply consumables ? I recall going to buy a MIG welder a few years ago, before they were hobbyist. We'd borrowed one, and had problems getting the brass feed nozzle. We were in the shop, selected a welder, went to the counter, and asked for 20 nozzles, which the sales assistant said "they didn't sell". So we left the welder at the counter, and walked out.