You'll be pleased to know that I've got it working. I think it's worth an explanation so here goes:
Originally the download behaviour was completely on the same page as the buttons that called the behaviour. I then thought about another site on the same server that had a similar download behaviour that worked! The difference was the fact that clicking the download button sent it to another page where the behaviour took place. When I tried to replicate that I had no joy and almost gave up but I then thought that I was using the site template to make the (new) page and then adding the behaviour.
So I tried it again but this time I just made a blank PHP page and only added the behaviours necessary (recordset and DFP). And it worked!
So it seems to me that the problem lay in just too much going on on one page. My template page really is a very busy thing.
See for yourself sermons_Detail.php?sermonID=125
So once again my apologies for all this but I really, really do appreciate the fact that you did try to help.