That was a great suggestion so I tried that today. Managed to get the top clip off but the pipe wouldn't budge no matter how much I coaxed it so it just went back together. Looks like that may have to be a job for next time it goes into the garage unless you have any suggestions on how I can budge that pipe. thanks for your help anyway.

which pipe did you try? It’s a 2inch diameter boost pipe. Should just loosen the jubilee clip and it slides right off.

I think I may have been trying to get the wrong pipe off so after your message here I looked again and found the really thick pipe running across the front of the engine, which I guess is the one you were referring to and the one I should have been working on. I took the top jubilee clip off and checked the inside of the pipe. I obviously don't know what a good interior or bad interior should look like, but while there was a fine film of black coating on the the inside, there was nothing I would have thought was alarming. Of course, your original message said you noticed the black soot where it joins the intercooler but I haven't yet figured how to get to that lower end of the pipe - I'm guessing I need to get under the car and remove the plastic guard panel to get to that bottom clip (unless I'm looking at the wrong pipe again of course ! Lol) to check the other end.

Yes that’s the one. Check it completely for cracks and leaks at the joints. Oily residue inside is fine. And yes you need to remove the under tray and the front guard. Quite a few little push clips and some bolts and it’s off easy enough. On the other side of the intercooler is another thick pipe you can inspect for cracks and leaks.

Managed to get all the plastic undertrays off this weekend to check the pipe and, sad to say, it all looks in fine condition - so looks like it's back to the drawing board, but thanks a lot for your suggestion

There are AWD dynomometers at some automotive businesses that normally cater to hot rodders. To maximze performance, most have OBD2 reader/controllers and exhaust analyzers to connect to a vehicle while it is running on the dyno. Your not-high-performance vehicle may also benefit from these tools/services. If I were you, I would call or drive to such a facility, explain the vehicle's problem and see what they suggest to diagnose it and what the cost would be to do so. (Have some heart medication and a defibrillator within easy reach when you ask for a cost estimate, just to be on the safe side.:))

Found the issue.
 

replace your Battery Current Sensor. It’s the little black box connected to the negative terminal of the battery. With a little plug under it with 4 wires. 2 of those to to the MAF. The other 2 go to the ECU. There’s a resistance test you can do to check it. If you want to contact me by Facebook I can explain the process to fault find it.

a faulty one causes engine surging and over fueling, poor maf readings. Poor charging and voltage fluctuations. Mine is faulty.