I have a site which is configured to use SSL and has a rule in the applicationhost.config to ensure any requests to http are redirected to https.
I then added another site to the box which was basically a copy of the first site and which I was using to test a different database engine. I realised that I couldn't put it under the main site because of all kinds of weird errors so a friend suggested that I add another level of subdomain so I could have test1.subdomain.example.co.uk and which I could then add as a header to iis to direct to my new site.
The only issue was that the server was configured only to allow ssl and I didn't want to buy another ssl cert for it so I tried to set it up to use the same certificate as the other site. This was the mistake I made since by doing this, it disconnected the certificate from the previous site without telling me.
Examination of the logs (after about 20 minutes when the penny dropped) gave me the clue that a request to port 80 was returning a redirect but then there was no request to port 443 which there should have been for the ssl connection.
All I did was assigned the cert back to the correct site and it was up again. Also, I then created a self-signed certificate for my other test site.