It's not unknown for the hydraulics to start leaking after fitting a new clutch, mainly due to the slight sudden increase in pressure needed to compress the new diaphragm/springs. Whatever is causing it was probably on the way out anyway if that's any consolation.
If you're losing fluid and you can't see where it's going, then look at the slave; if you're not losing anything (or it's going all over your left boot) then master cyl- obviously.
Slaves are worth cleaning up and a new seal- I always renew master cylinders. It's never worth arsing around with them IMHO. they always end up leaking again through wear. Cheap enough for what they are.
Running on is usually due to glowing carbon (occasionally wrong plug type), and what it's telling you is that things are overheating.
So, retarded timing, cooling system problems are the first things. The former is the most likely.
Very occasionally I have known this caused by slack timing chains- but not on Land Rovers