Hi again,
I sent an email to someone at the IoP. I didn't ask for permission to paste the email, so I will just give you a summary here: accreditation doesn't impact graduate study. She said that universities only care that the degree is recognised (which, as far as I can tell, every BSc at the OU is).
Accreditation, I was informed, has greater impact on your chances at a graduate job, such as engineering.
If you type, "open university mathematics and economics bsc accredited" into Google and click on the third link down, it seems your degree is NOT accredited (scroll down the pdf).
But as Persipan (and the lady at the IoP) have pointed out, this does not seem relevant to Masters study
So in summary, the university (OU) is 'accredited' (not the right word - recognised is) when it comes to granting degrees, and it seems that you can do it and use that for a Masters. I sent another email to her, so I might be able to give you some more information when I get the reply.
Oh, and by 2:1 Persipan just meant the grade; for the OU this means 70% or over. Most places either ask for a 2:2, 2:1 or a First. Realistically, you should be looking for a minimum of a 2:1.