Thanks, everyone. I did search for info on this but did not find much. Other than one vintage bike refurber who mentioned them. However, since posting this I have now discovered that Decathlon appear to sell these under their own brand Btwin. Now that may mean that at some time in the past I have replaced this cassette myself as we have a Decathlon locally. I don't remember doing it on this bike, but that doesn't mean it is not something I did a few years back when overhauling the bike.