Reservists, what's been your experience with your employer?

Hi all, I'm some way into an application to join the military reserves, and as yet have not broached the subject with my employer/manager. I work for a semi large company that does have a policy for this, an amount of paid days release for Reserve Forces commitments, but right at the end it says "subject to line Manager's discretion." I'm a little nervous about talking to them about it as my manager is notoriously difficult. For the most part I'm confident that I'd be able to accommodate it on my days off (shift work with varying days off etc) but there'll be times when I need to use some of this paid time for longer training periods. I'm aware of the legal requirements during periods of mobilisation, but for now the main thing is training. Can anyone who's a reservists tell me the reactions of your employer?