Hi WeeHughie,
It's been a while since I've visited the forum, and you've probably cracked this one by now! But just in case the following helps, I have found that home-saved seed from your own plants is a completely different kettle of fish compared to the seed you can buy in the shops. This is of course true of any seed, but it is especially true of stamnagathi which loses germination viability quickly (like all chicories, comparably to lettuce), within a couple of years tops. Also the seed you can buy in many nurseries and garden stores in Greece is frequently old and also kept on display stands exposed to baking heat in the summer and cold in the winter (although it is worth keeping an eye out for more specialised stores run by agronomists ['geoponos'/geoponika] where seed is kept refrigerated, usually for professional growers). You won't notice the drop in germination as much in seeds like tomatoes, but you will definitely see it in most of these kinds of greens (e.g. kafkalithra/mediterranean hartwort is another one to try and sow from home saved seed for the same reason). Stamnagathi germination is low to begin with and with old or poorly stored seed it just becomes impossible. WIth home-saved seed sown in a tray for pricking out, you will notice that you get clusters of germination: lots of seedlings in one spot, nothing in other parts of the tray. It's just uneven like that. Pricking them out in modules then helps you focus on raising the seedlings that do come up. I haven't tried the priming/wet paper route -- did it work for you?
If you are growing these in Greece, sowing times will be very different than what I say in my earlier post (you would be sowing in autumn). Mine have germinated well on a windowsill in the UK. Very high temperatures will clearly not help, as the plant's natural rhythms show, with seed pods drying out over the summer, scattering seed which germinates with the autumn rains. Cover seed only very lightly and do not overwater (again, same as all chicories/endives but also lettuce). It may help if you crack some of the pods in which the seed is encased to help free the seed and improve germination (this goes for store-bought seed too, which tends to come with the pods and other small bits of debris).
Good luck!