I forgot that actually, the coil and ignition module was to fix the 'only running on 2 cylinders' fault, rather than the small misfire that it's had for ages... So actually, I had fixed what I originally set out to, but it's left me with a fault it's had for ages, which I do need to fix... (massively down on power).
So anyway, some progress yesterday with this one...
After some discussion on FCF (I'll save the detail for that forum rather than here!) I have got Lexia talking to the car (partially).
First result was this;

NO FAULTS? WTF? You've had the bloody Engine management fault light permanently on for ages!
Anyway, I ploughed on going into the injection diagnostics;

Yes, well done, that's my car
Identification;

Errr...

Interesting...
Read faults;
Into this then, to see if that sheds any light;

I've no data to go on, but I thought the ignition advance looked a bit excessive?!
Anyway, the screen then crashed out (said connection to ECU lost).
Next up, to run through the test options it gives you (limited on this ECU)...

Great
Nothing worked from then on. I rebooted Lexia, turned the car off and on, no change.
Wiggled the leads around where they connect into the PCI unit and found the RS232 plug ever so slightly loose... Tightened it up et voila!
Suddenly we have logged faults! =D>

And the best thing? It confirmed my suspicion all along that it was probably a knock sensor fault!
Anyway, I ran through the rest of the tests;

All checked out ok.
The injectors are
really quiet though, took a lot of very careful listening - I was sure one wasn't working, so resistance checked them (all ok at 16ohm) and re-tested several times before I managed to hear the last one (difficult to get ear close up to that particular one).
So... Knock sensor. Difficult thing to test. All the wiring visibly looks intact. I'll bell it out back to the injection ECU and if that's ok, change the sensor. Hopefully that will be the end of it...
Citroen don't sell them anymore (not surprising - they just bin and delete stock en masse these days

) but readily available after market - next day into my local motor factors.