Looks like I may have kept the inner dimension a little too tight when I had the washer made. Fortunately the parent material being aluminium, meant filing out around a half a mill' wasn't a long or hard job.
My favourite goo, Plumbers Gold was used extensively to ensure a good water-tight seal against the mating faces of the washer and sink. The thread on the waste was wrapped with PTFE tape (a whole reel) for the washer to butt against, and help seal the thread inside the washer. More PG was added for good measure.... Although the bend on the trap is in poor shape, it looks to be capable of working for another 3-4 months, so that wasn't replaced. More PTFE tape has helped keep the worn/damaged seals working.
Bottom line, there are no leaks, and we have the sink back. Within days the sink proved useful, in fact I've used the sink for cleaning tools almost every day since, so I'm glad I spent the £16 repairing the waste.