how can I explain to a perspective employer that I am on sick leave
when job hunting?
I assume whatever leave arrangement you have with your current employer ends as soon as you are no longer employed there. Thus, your current sick leave has no impact at all on your next employer. You start there anew.
If I am incorrect, and you are planning to start a new job, yet remain out sick, then you must explain this early in the interview process. Few employers would have provisions for such a situation, so you must seek an employer who would be willing to accommodate your special needs.