nashorn

Hi Guys, Please can anyone help with this puzzle. I bought my M600 as a non-runner. It had a defective clutch. The slave cylinder was leaking. I have removed and checked the plates. None of them are sticking, all looks good under there, so I have put the clutch back together. I have bought a second hand slave cylinder from an M620. I have put the slave cylinder on today and successfully bled it. The problem is this - The lever is rock hard when pulled, with little freeplay, but it is not disengaging the clutch. I have removed the slave cylinder and could see that the cylinder was at the full extent of its travel. I am wondering whether the later cylinder requires a longer clutch actuating rod. Any ideas?
The original cylinder was the type that was assembled from the engine/actuating rod side. The later one is assembled from the outside.