Have I been unfairly dimissed?

I worked at a company for nearly 3 years, and was made redundant March last year. I was never given any firm reason why. Then I learnt 6 months later the company had hired a replacement for my job, and a month after that hired another person to do the same job (but a slightly more junior role).

Is there a set time duration before an employer can (directly) replace a position? And do i have a case for unfair dismissal (bearing in mind it's almost a year since I was made redundant)?

No specific set time limit, no. If they replace very quickly you could claim unfair dismissal on the grounds that the redundancy wasn't genuine. However the time limit for making an unfair dismissal claim is 3 months so if they replace at 6 months they would normally be fine legally (although possibly not morally), and certainly this long afterwards there's nothing you can do about it.