Are you applying to a top 20 (if you believe league tables) university? I got told they usually don't care about that sort of stuff as long as you have good predicted grades. I didn't write much about work experience from what I remember, just make it sound like you're enthusiastic about maths (ie don't say you want to do maths to get a good job.) And then fill the rest up with examples and attributes that show you would be an asset to the course (focused, will revise as much as neccessary, etc.) If you have read books on maths then write about that, but if you haven't don't read one for the sake of it, it'll sound fake when you write about it in the statement.