Thanks for your reaction Pastychomper. I agree with you to a point, namely, we don't know which
parts of the page are encrypted. We assume or hope that the vital parts ie bank details, are encrypted but we don't know.
I found some information from Firefox (the browser I use) http://mzl.la/MDvkxK
"When an HTTPS page has HTTP content, we call that content “mixed”. The page you are visiting is only partially encrypted and even though it appears to be secure, it isn't."
This would seem to be the case with myfavouritemagazine's subscription page, with a blocked content icon and the warning triangle icon.