I don't have a specific answer, but I did a bit of research a couple of years ago which gave me a rough idea.
The owners club used to publish lists of owners and the details of their cars, before the data protection act (1998) stopped such practices. Using the list published in 1991, I checked the car registration details of about 100 cars on the DVLA website -
https://www.gov.uk/get-vehicle-information-from-dvla - The list was a mixture of all models then available, so included some Mk3 coupes and a handful of convertibles.
Around 20 cars were either taxed, or had been SORNed within the past year, so those were the cars I would judge as being on the road.
Around 10 cars had been exported or scrapped - the DVLA doesn't specify which - and another 10 could not be found on their database, which pointed to them not having been correctly registered as a Midas.
The rest all had long outstanding tax liabilities (from before the introduction of SORN in 1999) or had been on SORN for a number years. So that means around 60 per cent of the cars that had been on the road in 1991 are still around, but haven't been on the road for sometime.