I wouldn't have a problem if they spot these mistakes before the game but after is outrageous. Just suck it up. There doesn't seem to be any regulation stopping them doing what the fuck they like.
"Sorry, your winning bet on Sergio Aguero FGS shouldn't have been 3/1, should have been 1/1000, here's your 0.1p back."
I'm very sure they would have refunded your stake if it had lost, on the basis that you wouldn't have placed it at 1/50.