I don't agree that it's the clutch: I suspect it's either the master or slave cylinders that have gone and/or on their way out. Doesn't do it from start when cold, but does when hot? Certainly sounds like that to me rather than the clutch itself, which would slip if worn but not stay down in use.
Everyone (Zmanalex included, and he is the god on these things) will tell you just to change the slave as that is the common one to fail. I would strongly advise doing both at the same time, as the part itself is only another £100 or so but it's the labour so bleed the fluid through that takes the time up and costs. Much for the same reason you'd do the flywheel as well as the clutch, I did the slave first and had to go back and pay a ton of money for the labour to swap the master out as well, should've done both at the time.