Some really terrible responses in this thread - unfunny quips looking to impress other users. If you have nothing productive to say to help, be quiet.
I feel for you, OP. I know how it can be when colleagues don't perform in the same way as you do. By the sounds of it, your managers don't help much and are also a cause of your stress.
I have been in your situation. If you have no proof of the theft, there's not much you can do, however, if you saw it, I would report it.
If your line manager or service manager is not being helpful, go higher. You must report the underperformance to your SM - but if that doesn't change anything, go to the General Manager or Deputy. This is what I have done in the past. You need to go higher up and put your concerns in writing. Be honest about what is happening and how you feel. Don't be afraid to do so.
Unfortunately, in my experience, not much will change in the long-term until you get different managers or leave the department. Managers can cover for other colleagues who are not performing. The NHS has cliques and this doesn't help. Good luck.