MSc in mathematics from MChem?
Just from a quick look at a few applied MSc programs, there are a number of classes that would be difficult to complete without serious background in mathematics: classes like partial differential equations, applied algebra, probability theory and combinatorial algorithms were a few that showed up in most that I doubt you could manage without rigorous undergraduate preparation. However, if you have taken classes to prepare you for this (like a minor as an undergraduate), or the MSc you have in mind is less rigorous, or you are willing to devote a year or so to prepare on your own, I fail to see why they would refuse you to take the entrance exam. At least in the US, a good subject score on the math GREs would show the university that while you have a less traditional background, you can still do well in the program.

Basically, it comes down to how mathematically rigorous the program is and how strong your math background is. Liking math is unfortunately not enough to pass a class on PDEs. You would probably need to provide some evidence in your application that you have a sufficient math background, and taking a standardized exam with a good score, even if it is not required in the country you are applying, would be one way to do so.