As there was no final resolution to this thread, after tolerating a low biting point and heavy clutch pedal for over 3+ years, & 30k miles, last week following a gearbox disaster, I had a new clutch fitted at the same time.
The instant result, using the same clutch cable etc was a normal bite point & feather light operation (relatively).
The friction plate & thrust bearing were still in serviceable condition, so the root cause is a gradual deterioration of the diaphragm spring in the pressure plate, resulting in excessive pedal movement before the clutch freed off, and an inefficient operating angle resulting in a heavy pedal.
Sorry but clutch replacement is the answer to this problem if you cannot live with it.