Mark, my fave remedy for furball is tinned fish, like tuna, in oil. Drain some of the oil away, but give the rest to the cats mixed in with the fish. Sam gets this about every fortnight.
A furball, when thrown up, should look like a mouse, or a sausage of fur, and be fairly squashy. Sam throws them up frequently. She washes herself, shorthaired, and Polly, fluffyhaired; so swallows more than her share!