Let me think...
at 20A and 230V 1ph for 100m I guess you'd end up somewhere in the 6-10mm2 range to keep voltage drop at an acceptable level. For 3% voltage drop I get 10mm2 and I really wouldn't recommend any more than that voltage drop since you can have as much already in the fixed wiring, i.e. a total voltage drop of 14V under full load - could be noticeable with lights and you'd heat the air with your cords. However, that's assuming full load on each cord.
I guess your second best bet is to combine several lights of no more than say 500W each and connect each to its own 2,5mm2 extension cord. That should keep voltage drop within acceptable levels and the cords should be easy to get by. Don't piece together short pieces though, use two 50m pieces max.
It looks like the board at the house already has RCDs (though I don't know how they're connected), connecting another RCD at the end of the extension would cause problems in case of tripping (either both trip at once or either, unpredictable which one). I'd recommend to check what the RCDs control and only use those outlets.
