Some good news. I went over to Pick A Part part in Kilsyth www.pick-a-part.com.au/ and found not one but several Volvo 240s... mostly from the 1970s and 1980s. They had the correct tail light and I pulled out several just to check and compare. It really was a much of a muchness. They were all dirty and covered in mud. I choose the one that had the least amount of cracks and was the closest (in years) to my car.
Bearing in mind that my mechanic wanted to charge me $120 for a second hand tail light- I was prepared to pay $50 for it. I was greatly relieved that the guy charged me just $21 for it.
Hey, wait a minute!! I could make a business out of this! Does anyone want a tail light for $40?? lol.