I'd be very surprised if most universities would normally consider anyone for a PhD in Mathematics unless they'd got a 1st (or possibly a 2:1) in their initial degree (in Mathematics) and had excelled (again in Mathematics) for their MSc.
However a doctorate in Mathematics will always be a research degree, rather than a taught degree. If you could convince a university that you've got some novel ideas for developing the subject, together with sufficient knowledge to do so, it's just possible that your application might be considered. For example if, through your own studies, you'd already discovered some interplay between field theory and chaos theory which you believed nobody else had thoroughly examined, you might get the green light to study it for your PhD.
However you'll need to be able to show that there is plenty of 'depth' within what you want to study. One standard proof in current books on complex analysis was discovered by me. (I'm actually credited with it in some volumes). However 'one swallow doesn't make a summer'. There's no way that I could ever undertake a PhD in Mathematics.
Chris