well as this has been a long-standing problem i have tried lots of stuff (most probably irrelevant to the problem, but hey i tried, right?):
- cleaning AAC valve
- cleaning AFM
- adjusting TPS
- re-grounding fuel pump
- cleaning carbon off the O2 sensor
- changing fuel filter
- reset ECU
- checked coil packs (although 'Roy' suggested this is better done when they are warm)
and for the record, the O2 sensor was an NGK one with the part number "0A08" on it...
Waz.