As I said in my original post “I’ve no recollection of entering my own details.” so it is quite possible that I have forgotten doing it. So, I decided to revisit the site to go through what I’d need to do to enter my own details to see if it “rang any bells” for me. Unfortunately, just before I posted this response, the website was inaccessible (via IE, Firefox or Chrome). Maybe it’s down for maintenance but the mystery must remain unsolved for the moment.
UPDATE: The site is now acessible and I have gone to the “Add a Welsh Marker” link which does indeed look very familiar to me, so the probability that I had simply forgotten rises from 50% to 99%
I apologise to readers of this thread and owners of the website for possibly raising any doubt. I am, needless to say, embarassed. 
