Slave cylinder mate. Had exactly the same myself a few weeks ago with the same kit - Helix Organic clutch, LUK DMF, new slave, all fitted a couple of years ago and probably done 10-15k since.
Apparently the slave cylinder isn't of the best construction and doesn't like heat. With the Helix being a heavier clutch than OEM maybe that contributes to its demise.
But yeah it's box out for you I'm afraid.