Posted this question a few days ago and I thank you for the replies and hints received.
I am reposting as I have fresh info.
Story as follows;
I approached a junction, pressed the clutch pedal, it felt far too soft and I could not engage a gear. When I let my foot off the pedal it stayed down!
Anyhooooo I put my toe under it to lift it and after that it was ok!
I have thoroughly examined the whole system as far as is possible.
No leaks are present and no fluid is being lost.
The system has been thoroughly bled through, no air found and it now has lovely fresh Dot 4 fluid in it.
The spring on the top of the pedal appears fine (later type, coiled around the pedal pivot shaft) appears fine.
When bleeding the system, after pumping the pedal - holding it down - releasing fluid from the bleed valve the pedal stayed flat to the floor every time and had to be pulled up!
I am thinking the following;
Fault in master cylinder, maybe a folded seal that is allowing pressure to be applied but not sucking fresh fluid in on the return stroke, this in turn would hold the pedal down against any vacuum created.
Any ideas anyone??