Latest thoughts:

1) From the point of view of testing the O2 sensor, there is a great post from Arthur Dalton on this forum to the thread "Oxygen Sensor Voltage Question", where he gives a really good technique for checking if the sensor is working correctly. Will give it a go sometime soon.

2) Read up a bit more on fuel system cleaners and the one I'm keenest to try is Red Line's SI-1 (or the smaller bottle called SI-2). Seems to be as highly recommended in the US as Techron, plus is available in the UK - even if it's 4 times the US price! (i.e. 15oz bottle in US lists around $5 while the equivalent 443ml bottle is £12.50 in the UK).

3) Got the car misbehaving nicely last night (very rough idle at the end of a 15min trip after the car had been on the driveway for 3 days). So had a chance to try out a tip I read somewhere: Left the engine running; opened up the bonnet; took off the cover to the top of the leads/plugs; one at a time pulled off (then replaced) the plug leads and listens for any differences in the engine's idle. End result was that for 5 of the leads, the engine ran even rougher when they where 'popped' off, but for the other one (cylinder #1) it made no difference if it was connected or not. This I presume is quite significant and points to my problem being with the firing of this cylinder, so dizzy cap, lead, lead-end, plug and injector are the likely suspects......

4) 'ericgr', pretty sure car does have an ICV. Cleaning it plus checking its connections is on the list to try if my cylinder #1 investigations go nowhere.

5) Also on the list was to check out the fuel pump relay but initial investigations have failed to even find it! I thought the 3 places to look where behind the battery by the OVP, behind the main fuse box and under the rear seat - all checked and no joy! Did read somewhere though that for some cars around '92 its functionality is included within the Engine System (MAS) control unit that in my car sits behind the battery along side the OVP, CIS-E control unit and HGS control unit (for the 5-speed auto). Anyone know if this is true?