Well, I have discovererd the problem (or the cause anyway)

Its not the MAF, (The MAF may be a bit less enthusiastic than it might be, but it is not the cause of the problem)

It is water in the fuel!

The OSR injector (the one nearest to the bulkhead) is at the low point on the injctor rail. I am guessing that water entering the rail "Pools" at this point producing a misfire on that cylindr. The ECU registers the misfire but can not "see" any reason for it so eventually shuts down the injector on the offending cylinder to protect the engine/cat from possible damage. and generates a non-specific fault code.

How I get the remaining water out of the system remaints to be solved.

But at least I know what I am dealing with now!

(As always, this fault only manifsted itself on the day the owner had decided to trade it in for another car, so he doesnt really want to spend much on it!)