Actually I don’t know if I did. Grasping at straws I added a Feedburner plugin and…mysteriously…everything seems OK now.
this one: http://flagrantdisregard.com/feedburner/
However I’m not sure why this made a difference? I’ve been using feedburner (without a plugin) for months without this happening. I haven’t been doing anything on my website for days. So I’m perplexed as a) I don’t know what happened or b) why this seems to have fixed it.
ps. Again - thanks so much for your sage advice