Thanks, will keep that in mind. Never occurred to me that that the valve can be the culprit. Doubt it's that though as I replaced that complete unit not too long ago. My issue also seems to occur even when the pump is not running and since the valve is much higher than the pool, that shouldn't be an issue.
That said, it's been something like 16 hours now and the water level dropped by 1cm. This is in a 8x4m pool so it's ~320liters?!?! BUT, the water level in the bucket dropped even more, so it could be evaporation. I took the solar blanket off for this test.
Only about 1cm to go before the water will fall below the weir entry, so going to keep it going for now.