306 clutch help please

Hi
I noticed today that all of a sudden I had great difficulty in selecting
gears, Its been fine till now, If I let it cool down I can select gears and
drive away fine but then when it warms up a bit the problem reappears.
I opened the bonnet and I have the self adjusting type clutch cable and I
could pull the *slack* cable out about 4" from the clutch fork. I got a
friend to depress the clutch pedal and tried to lever the clutch fork
further in but it would not go any further.
When the engine is stopped I can select any gear with ease.
Am I right in suspecting the clutch needs replacing or is it some other
problem. The reason I ask is because I have had no clutch slipping or any
other sort of warning at all
cheers
Rikk