Big companies usually pay well. The ones you may find not paying well would be some small independents but fortunately not many. It is possible to take people to court, but that it is expensive. You should consider if the amount of money that is owed to you is worth it.
I think it is good practice to mention the names of those naughty practices in forums like this one as well as to report them to AOP and College of Optometrists and the Ombudsman. If they contacted you through a locum employment agency lets them know about the situation. They will stop providing locums for him.
I do not know if you will get your money but at least we can try to close the circle for those cheeky b…..ds that want to take advantage of your hard work.
One more thing. If you do not know the practice because it is new to you, do not take many bookings at once when you contact them for the first time. Do not put all the eggs in the same basket.