Duh - I just discovered the feedback link.
If you've left a comment in the past and wondered why it wasn't published when others where - here's the answer!
I've had auto publish on for users who have an account and it's been on "moderate" for anonymous comments. I just found where to moderate the comments - and have published all the unpublished comments.
Isn't there some sort of award for dumb things like this?
And my apologies to those of you who feel that I've been ignoring your comments! :-)