Do I have a case for unfair dismissal?

It is unlikely that you will get dismissed while on sick leave but when you return to work they can dismiss you because your work was not done correctly, not that this is true but it is their word against yours, you will be better finding another job as you are fighting a losing battle. I have worked for over 30 years and you always have people like your manager in the workplace and it is something you just have to put up with. Good Luck.