The Doncaster AA site does not state the size of the fish, it says that its a large barbel. For the river Don there is no doubt that it is a large specimen irrespective of its weight.
I only know of one 10lb+ barbel from the Don that I believe to be factual, but there are also a lot of discreet fishermen the same as anywhere else who prefer to keep quiet.
Don't forget its not long since the water at Sprotborough (and almost everywhere else) was too frothy and scummy to be seen, never mind fished.
Also about 4 years ago we had a serious amount of water flooding through which I believe lost many established fish to the lower maybe even the tidal stretches. Even now the big bream (5lb+ fish) are a rare sight at the back of the prison and the barbel population has not recovered to the previous stocking density.