This is a nightmare
It used to be called ESA employer support allowance.
I had an op about 5 years ago and had to claim. You have to claim on the phone and it took about two hours. My books had to be up to date with earnings and expenses
they take everything into account then they awarded me 70 pounds per week! You cannot backdate claim starts with first phone call. As soon as you do one job back payment stops.
Waste of time in my opinion.
Since then I have had three operations. (I am recovering from latest at the moment probably another three weeks till able to start back again) and I have not bothered claiming. I have just had to rely on savings.
By all means do not take my word for it because the system may have changed.
All the best.
Colin