In my mid 40s I was able to get onto, and do well in, a conservation MSc after a History degree, some history/politics postgrad study and 10+ years practising as a solicitor. As you suggested, I did a couple of maths/science OU modules (S104, M123 and M140 would be good) to show I was still capable of study and to get used to maths/science. Sadly OU modules are much more expensive now than they were then. I also had a fair amount of relevant volunteering experience. Once I was on the course I found that I had much more natural history interest/knowledge than most of my cohort.
As a specific example of a bridging course, UEA do a postgrad diploma in ecology that feeds into its Applied Ecology MSc
I agree that you should contact some unis/course leaders to see what they say.