Thomann out of Germany has stock oem ear pads.It worked so thanks very much for the video. There are 5 lugs that 5 corresponding holes on the earpads need to engage. An anti-clockwise twist of about 5mm removed the earpads and the reverse fitted the new ones.
Now I just need to work out how my new Chinese leather pads compare to the stock ones. They are a different design: more a traditional pad rather than the "hollow earcup" of the stock design. They are also a bit thicker. Initially I feel I have lost some of the soundstage I love about the K812s but that was based on a hasty session this morning.