Well according to the avt007 booklet, if a voltage reading from the engine (meter leads to throttle body and shock tower with engine running) is anything above 0.2V it is an indication of a bad engine earth connection - I was guided by this tract as I know next to nothing about electrical troubleshooting, if thats wrong then who am I to know !? The earth strap itself "looks" ok with no corrosion but .......
So far the no start scenario has not re-occured, I am currently of the opinion it may be the ignition switch not engaging, or disengaging when switching off , intermittantly. The tests I have done so far do not indicate a definite "problem" with the electrical circuit but I have very little knowlege of this area.
The workshop manual states :

When the ignition key is set to the normal run position, the following sequence occurs:
 The isolation relay in each fusebox is energised to supply battery power to the fuses and associated circuits.
 The BPM 'IGN ON' terminal is taken to 0V by the ignition switch.
 The BPM checks that various parameters are correct e.g. - inertia switch is in the active condition - transmission is in
PARK - security system has been correctly disabled.

Whether this throws any light on it I'm not sure....

Thanks for all input though, even critical coments can be of help!

AndyP