Did you ever resolve this? My best guess is a loose N conductor in the meter / cutout, if everything else is ok.
Some people also presume they can test an appliance at the plug, but I'm sorry, a cooker, washing machine, dishwasher etc, have heating elements and motors that are only energised through a relay when the machine is operating. The ONLY way is for a clamp meter on the earth to measure earth leakage when these appliances are cycling through their operations.
I once had a dishwasher causing the RCD to trip. It was a break in the cable linking the door to the main body (I presume) as we managed to find it tripped in a certain position.
Love to take on your job but too far for me and you MUST have had it sorted by now. RCBO board would certainly help with this.
Also - final thought. You could have a shared / borrowed Neutral. You say you had an electrical test, but whoever did it failed to note the alien MCB in there, so I doubt he has done a thorough job. Has anyone undertook continuity / insulation resistance between all the neutrals? It's seldom done but very important.
