Do I need a mainsheet block?

Greetings all, newbie question if you don't mind.
Twice now we have tried to take our DS1 out and twice the mainsheet line has jumped the pulley at the top of the mast. This jams the cable between the pulley and the housing of the block. There are two pulleys up there and one looks aftermarket with too much play in it.
I see D&R marine has a "Daysailer Mainsheet Block Only" for $75 but there is no picture. Is this what I need to fix the problem?
Thanks in advance.
Twice now we have tried to take our DS1 out and twice the mainsheet line has jumped the pulley at the top of the mast. This jams the cable between the pulley and the housing of the block. There are two pulleys up there and one looks aftermarket with too much play in it.
I see D&R marine has a "Daysailer Mainsheet Block Only" for $75 but there is no picture. Is this what I need to fix the problem?
Thanks in advance.