Hi Brian and Willi,
Thanks for your welcome messages! Willi, I had a good look around the site and did see your Vulcano - it seems especially hot!
Brian, thanks your advice - I have now looked at the access nut on the bottom, and on mine there is no date, no marks at all, neither on the top or the flats of the nut; and nothing on the bottom.
I have just three days ago got another one - this time a Primus 820, in worse condition, but only £2.00. I will clean it up and show it here later.
Best wishes
Andrew