Thanks for the input and suggestions so far guys / gals.
This thing is haunted, I'm almost sure of it.
I have checked and measured all of the voltages and resistances as suggested.
All are fine, except the pin 10 output from the LM723 and pins 2 and 3 which monitor the voltage across the current sense resistor.
The tracks to the TIP31c driver etc are fine too.
Tested C25, no problem there so replaced anyway, J1 diode also OK, replaced anyway as I have a pretty good stock of bits acquired from many hamfests
What I have noticed is that the fault varies, in that the output voltage will occasionally be able to be adjusted up into maybe the 9 volt range, and that its also unstable, which makes me think something is breaking down under load.
I decided to measure across the output terminals, don't ask me why I hadn't thought of this before.
It reads 245 ohms either polarity, hmm, that doesn't look right to me.
Looking at the circuit, there is nothing that accounts for that resistance, I even have the LM723 out of its socket when I measured.
I'm off to the shed later so I will report on my findings later.
I'm retired due to Ill health so can only tinker when things are going my way a bit