Solar Inverter causing earth leakage trip at startup

[geo3geo]

On 26th Jan I had my faulty loft PV inverter replaced. I opted for a Solis 1.5KW as it's the same as I installed myself on a garden array a few years back. The next day, about 8:30am the mains tripped on earth leakage. I reset it and everything was ok. Next day, the same again, and the penny dropped. I examined my weatherstation logs for solar intensity and sure enough the trip was just at the point the inverter started to export. 

For 10 days now I've been trying different combinations of isolating supply loops to home in on the problem.

I'm now 99% sure that the problem is some sort of  interaction between the two inverters. If I unplug the garden inverter from the mains there is no morning trip. Connect it and there's a trip.  And when it trips, if I reset it straight away it usually trips again 5 minutes later when the loft inverter starts exporting again. If I reset it a second time it stays reset.

The fact that the inverter is fine on the second trip reset implies it's not a high residual earth leakage issue.  To be sure, I've isolated the garden cable run (underground armoured) and unplugged everything and checked E-L and E-N resistance - no obvious issue.

Does anyone have any bright ideas?

Thanks