I've done S111 and MST124. S111 is only an introduction in terms of the maths.

MST124 is like the first year of A-level maths (so I've read).

So my recommendations are do the "Are you ready?" quiz, get a list of the units and if necessary buy an A-level maths book or the MST124 books used off ebay (that's what I did).

https://students.open.ac.uk/openmark/mst124.ayrf/

I found MST124 more challenging than S111 and had to do a lot more work for it. The more prepared you are, the easier it'll be.

As with all maths, it's all about practice and you'll pick up the concepts more easily.

Good luck.