I think problem solved... and it may have to do with the fact that I had not initially entered a callback url.
I tried it with the widget from the drop-down and it was the same problem. So I decided to delete, via CB User Mgnt, the twitter_userid from one of the test accounts and re-link from the front end. It appeared after doing that.
So I'm going to run a query on all the registered users that had used the twitter link facility in the past (shouldn't be many), delete their IDs and ask them to re-link also.
Watch this space...
Ess.