OK, here's the admission - I did start up the test with the TA3020 installed but not powered up - don't ask how! Then got it right but V5 current seems a little high at 60mA but the VN10 current does what it is supposed to - i.e. small inrush then settles close to zero. Then connected the VNN with 50 ohm protection and it got very hot very quickly - it draws around 500mA (didn't bother with VPP).
Took the TA3020 out shorted the FETs gate to source and retested. All measurements correct. I cannot see or test any obvious shorts. I even took one FET out and compared it with a new one and all measurements seem OK but is this accurate? Can a FET be tested a) in situ and b)without a scope?
Have tested the TA3020 in another working Truepath and it is fine.
All suggestions welcome!
b.t.w. when searching for the data sheets for IRFP4321 Google kept directing me to IRFB4321 which is very similar but can handle higher current - interesting!