"Mains plugged in while engine running"

70 plate Fairford with Sargent EC700 control panel.  Now showing message "Mains plugged in while engine running" and alarm beeping virtually non-stop.  I've turned all fuses off then on, and gone through mains connection and disconnection steps as per the manual - but still the problem persists.  Any ideas how to resolve please?

Have you actually unplugged the mains lead from the van? Just being connected causes the alarm

Yes, and driven it several miles but still the problem persists unfortunately

Just guessing, have you completely shutdown/reset the EC700 at the main unit?

I turned the Power Supply Unit (PSU) off and for the hell of it the main fuses too, then put them all on again - so I presume that constitutes what you've asked.

Were there any other changes to the van, or do you get any other symptoms which started at the same time as these?

On the control panel does the lighting strike mains icon still show ?

and have you tried turning OFF the orange and green buttons on the front right of the power supply ?

No other changes to the van and no other symptoms have developed.  I'll have a look next time i'm at the van.  It's stored on a site a few miles away.
Not sure about the lightening strike mains icon.  I did turn the orange & green lights off and then on again - or whatever the 'un-hooking' instructions say.  But still the problem persists.
Thank you all for your persistence in trying to identify.  The chap I spoke with at AS was at a loss as to what to advise.

Then you should call Sargent tech support and have a chat with them, it does sound like there is a problem with the main power supply unit as the mains is turned on by the green and orange switches. They can send parts ahead to your dealer if they can identify what the issue is.

Thank you very much for your help