No one pitched in on this one, and as it happens - it looks like the problem is a poor shore power supply after all. I found that with a 3kw load, shore power volts dropped to 192 - 206v, Then I had to move over to the other side of the canal where the Chantier Naval operates (Saint Valery sur Somme) to get water, and while there I found that their shore power dropped to 182 - 192v with the same 3 kw load!
I have a Mastervolt 24v/4000W inverter charger that is probably around 12 years old. In the last few weeks I have noticed mains-powered lights flickering occasionally, certainly when shore power cuts out, but recently even when shore power was on and the batteries on charge. The recent phenomenon was a low frequency (i.e. very visible) stable flicker that lasts for about 3 seconds and repeats itself intermittently. Later I noticed that that the microwave had lost power, and after a quick test found that the line voltage had dropped to 206v. Then once shore power was off, the voltage reading went back up to 230v.
The inverter panel has one switch - CH/OFF/ON and I have always left it ON under normal circumstances. Has anyone out there had these symptoms, and is it indicative of an impending equipment breakdown within the unit?
I do have the laptop connection and software to get into the system, but not the knowledge of how to go about system diagnosis, if that is possible.