Thanks for your answer!
Well, I identified the specimens I collected with Peck and Boucek (Keys to the Chalcidoidea of Czechoslovakia), and indeed I ended up with gigas.
The basal funicle segments are longer than wide and reddish, in intermedia they should be as long as wide and black (I think I also collected those elsewhere on the island). Furthermore they are very big, about 15 mm. But I'm by no means an expert in this family, so I could be mistaken.
I can make pictures of the collected specimens (6 females) and place them on the forum if needed.
Peck and Boucek also stated that there are only females in Europe and so the species is parthenogenetic, and indeed that is why I wondered about this clearly hilltopping behaviour.
Whether they enjoyed the sun I don't know, maybe they preferred the bit more cooler mountaintop, as it was very hot (40 degrees celsius) on the lower parts. On the other hand, the mountain is not that high (756 meters) that it is really much cooler.
Near the top stone of the mountain (first picture) they formed a group of sometimes 50 to 60 animals, sitting on the stones in the sun or flying together.
Around that top there was a rather big flat plane that forms the mountain top, and there you could find them everywhere on the stones and bushes, whereas lower on the mountain I found none.
I forgot to add that when I visited the mountain 2 days later it was more windy on the top and all of the Leucospids were gone, I saw not a single specimen.
About bees, I remember having seen some Megachile, but as I'm not a Hymenopterist I don't know what species, and I didn't collect them.