I've done two drains and refills over the past few days, the second one had 300 miles on it. Last night did the third drain and dropped the sump to find it largely clean and a genuine GM filter that was manufactured at the end of 2005. So even though there was nothing in the service record to say so, it's obviously either had a flush and filter at around four years old, or a whole new transmission.
I'll be doing the drain/fill/drain/fill/drain/change filter/fill every two years from now on so hopefully it'll last another 120k.  2003 Vogue TD6/1992 Classic 300TDi/2x 1991 Golf GTI/1966 Volkswagen Kombi/2x 1968 Volkswagen Kombi/Probably something I've forgotten...