Finally got the idle hesitating last night, but when connected up to the multi-meter the readings where as expected - fluctuate around 0.15V - 0.85V.

My assumptions from this are: 1) The O2 sensor is working correctly, 2) the hesitation is not being caused by a too rich/lean mixture.

Things might be different when the car goes into its stall/non-start mode, but this happens so infrequently I'm going to tape up the wire, put the flooring back down and look elsewhere for now.

Next things I was thinking of doing/checking:
a) Hard drive with Techron Concentrate added to a tank of fuel (as suggested by 'ericgr') - only problem there though is I haven't yet found a supplier in the UK. Anyone know of one or a suitable alternative UK available product?
b) Fuel pump relay (was reading today about possible bad solder on the pins)
c) Idle control valve (check the hoses for a leak)

'dtf': did see your thoughts on Air Flow Meter (also called 'Mass Airflow Sensor'?), but am pretty sure you don't get one of these on a CIS-E (KE-Jetronic) injection system.

'ericgr': Plugs are as specified in the manual - I'm pretty happy these aren't the cause of my problems

If I get any further results I'll endeavour to update this thread so as to hopefully help others.

Cheers for now,
Neil