I've just uploaded all the files to perform the upgrade and ran upgrade.php, as my website is using vBulletin 3.5.
As I already had 5.14, I just ran the 5.1x - 5.2x upgrade option.
I get as far as "Preparing to update database..." on the first step and it just stopped.
I then went back, and followed it through again (thinking I had made a mistake) and now get the following :
Content visible to verified customers only.
This, I assume is where it has already conducted this step on my previous attempt.
In both cases however, I do not appear to have a completed statement or a continue link?
I'm open to ideas