Have a look at my post one page back, post #3139. The symptoms you describe are typical of there being a copy of El Capitan installer app already on your machine.
Thank you; was the first thing I tried. (And has worked repeatedly before.) However for some other reason it is not working now, even with all non-boot drives pulled and the Spotlight cache rebuilt on the boot volume.
It's going beyond the 'Download' button in the 'Purchased' tab, and taking me to a dialogue box inviting me to choose between download the installer or cancelling. The spinner starts after I elect to download.
When I go to the El Capitan page via an iTunes URL in Safari) again it's failing.
I had just installed last week's Security Update, so wonder whether that changed something?
Last edited: