My prostatitis has gone following treatment. Is it likely to come back?

Hard to say... But don't borrow trouble worrying about it. An issue with prostatitis are the annoying urinary symptoms, but also the psa issues. This is more an issue after seed implans than external beam in my experience. Usually when these flare up, and then subside they do not come back, but medicine is not that predictable!
YES. Non-bacterial chronic prostatitis is just that, chronic. It can flare with dietary irritants and stress (both mental and physical) . Maintain a healthy balanced diet with regular excersize and it will flare less and less often as you get older and wiser.