Why not go to your nearest O.U. Regional centre to look at the course
material for all astronomy courses and see if you are happy doing the
maths involved.

My advice is, if you want to get the most from the courses, do the
ground work first.

I have no problem learning the maths and physics, just naively hadn't
expected it. I do seem to remember doing A level maths was probably one
of the most intellectual things I have done (that was when an A level
was an A level), so it will be good to refresh myself on trigonometry,
calculus and so on, and actually put it to real use. Eat your heart out,
Isaac Netwon (maybe!).

A good suggestion though, I'll ask someone at work who has just finished
an OU degree where the regional centre is etc.