I am a bit late answering, but I am also a current Q31 student so perhaps my specific knowledge of the course can help here.
I will prefix this answer by saying that in 2014 or so, the Open University decided to "modernize" its courses and one effect of this was to mandate 120 credits at level 1, not letting you move to level N+1 until you've done level N, as well as dropping things like Topology and Mathematical Logic as possible separate modules. They're moving to a scheme of mostly unavoidable 60-credit modules that cover everything to a reasonable degree, with far less flexibility. I don't like the changes, I believe the OU are dumbing down the maths degree and more broadly I am not alone in disliking the current direction of the OU.
Ok, contextual rant out of the way. I started on the previous degree programme B31, which only mandated 60 credits of level 1 study. I filled in 30 of these credits from the worthless "using statistics" module and have an exemption to use level 3 30 credit modules as level 1 modules. I suggest you ask for the same thing.
What this means is that you will be paying more for your degree and further, that you will be doing harder work for credits that only count in the sense of completion but have no weight for your overall grade. On the other hand, that for you might be (and for me definitely was) a worthwhile tradeoff instead of doing something mind-numbing.
I can't promise they'll let you do this and they certainly won't like it because "distance learning is difficult" (it is) but if you feel you are up to it I would insist as much as you can. You won't be able to replace the whole of level 1 either, but you might be able to get out of doing the full 120 credits. I suspect they will likely want you to do Discovering Mathematics, which I suggest you blow out of the park and then ask to move onto the level 2 modules, with your intent to substitute some of the level three modules for level 1 ones at a later date.
To give you an idea of topics covered, at level 2 you will do M208, which covers:
- Linear algebra
- Sequence, series and Real analysis
- Basic group theory
- Some foundational material needed for the above i.e. limits
- First and second order ODEs.
- Vector algebra and statics.
- Basic motion physics.
- Matrices, eigenvalues and eigenvectors again).
- Linear differential equations.
- Physics like oscillation, damping, modelling, multivariate functions, rotating bodies, angular momentum.
- Vector calculus
- Fourier series
You then have choice of level three modules in the mathematics curriculum, which is where the fun actually starts since much of what I've listed above is basically first year and maybe second year mathematics for many full time undergrad students. The modules at level 3 are generally quite in depth and quite good and the material is well written, if a bit repetitive at times (useful though for distance learning).
So I guess it depends how much this material matches the level you want to read mathematics at. You should also realize that even if the course isn't that hard, the mechanical work of completing the exercises, assignments, exams and studying the material is quite time consuming. I am sure you can do it, but you should plan on this taking you 6 years to complete at 60 credits per year. If you are organised and only care about passing, you might manage more than this with a full time job, but it will not be easy.
Since you really seem to be looking for the content rather than the actual qualification (since you already have several degrees!), if you want to get up to speed with the sort of material on MST210 it might be better to buy a copy of Jordan & Smith's Mathematical Techniques first, which is "the" reference textbook for, well, mathematical techniques, for maths, physics and engineering degree students as the book says. It'll give you the basic grounding for the applied side of mathematics up to about second year mathematics undergraduate in the UK for most courses if you read the whole thing (you'll miss out on the more theoretical courses, which will likely be real and complex analysis, group theory, ring and field theory, number theory and you'll also miss out on statistics).
On the other hand, if you already have this grounding, you won't get beyond it until 240 credits into OU study and I'd second other answerer's comments about finding a suitable masters programme as a probable starting point.
The Mathematics Institute at Oxford also helpfully publish all their course summaries and most importantly of all reading list recommendations which are pretty good. If there are specific areas you need to know about at a more advanced level than Jordan & Smith, this is a good place to start looking for book recommendations.