Ahah! Now I can see the problem.
In my CHurch theme [I use an old version] I have under
Appearance/Revolution Church Options
a box to insert code 468 x 60 between the post and the comments and someone [I had someone tweak the site] has replaced that with the 600 high sidebar code for adsense [which is still on the site in the right position] I have now removed the 600 high one from the box for the 468 x60. Now I need the code for that position which came with the theme.