Thanks to both people who responded. You got me pointed in the right direction. For the benefit of people who might have a similar problem the resolution was this;
When the cart opens a new page (or image or whatever) it uses the configure.php files to create the url by combining your HTTPS Server definition (i.e. HTTPS_SERVER as defined in line 17) with your catalog directory definition (i.e. DIR_WS_HTTPS_CATALOG as defined in line 26). My includes/configure.php file was as follows;
Line 17 - define('HTTPS_SERVER', 'https://www.mywebsite.com');
and
Line 26 - define('DIR_WS_HTTPS_CATALOG', 'https://');
I don't know why line 26 was like this. I'm pretty sure I never touched it before so I'm guessing that is the default.
so when you concatenate the two definitions you get;
https://www.mywebsite.comhttps://
This is obviously an invalid url.
I changed line 26 to;
define('HTTPS_SERVER', '/mycatalogdirectory/');
and everything worked fine.
My thanks also to whomever commented the code in the admin/includes/configure.php file. The comments were really helpful to a non-programmer like me in figuring out what was happening.



