PhD Maths with a 2:1?Watch
Need more help on going postgrad?
That said there are some projects where funding and admission are tied (usually pre-determined projects with funding already attached to them), which may stipulate a 1st is required to be considered (but may say a 2:1 is the minimum for consideration). Also sometimes PhDs are offered where the funding comes from a fellowship or similar awarded to a particular researcher/research group; in these cases sometimes the awardee has more power in deciding who gets funded for their projects, and so may be more flexible and if you are a good "fit" for their project/research group, might be keen to take you on (especially if its someone you've previously done research work with)
Strong performance in your research oriented elements of the first degree will probably be good for convincing the uni to admit you, but may not be sufficient for convincing funding bodies. Getting funded ultimately is the biggest bottleneck, and may end up being a bit of a coin toss situation for you without a 1st. I would recommend you explore options far and wide, and contact potential supervisors to discuss your situation (as well as matters about the actual project itself, or your proposal if it is not a pre-existing project) and see what they advise.