No job is worth it if causes you to gamble or to be unhappy.
Even for half the salary, it would be better to change.
Would it be best to get your health in order before you start a new job? In that way you will be less worried about looking for sick leave when you are still new.
Why not take out Payment Protection on your salary. Can you do that through your Union? It would be one worry less when you are waiting for your surgery.