Everlasting sweetpea - when to sow?

For the perennial sweet pea, I suggest you plant the seed exactly where you want the plants to grow - they don't like root disturbance anyway, and I'd do that as soon as you get there in April. If the one you're growing is Lathyrus latifolius, I reckon that'll survive the winter, not sure about the other perennial ones.
As far as the sweet peas (annual) are concerned, you have no choice but to sow those immediately you arrive there in the spring - just means the flowers will be later, and they're better in the ground than in a pot - but I think you've discovered that already!

Oh that's a relief - I hate messing about with seedlings in pots, let along transporting them across Europe! Yes, it's latifolious. I might just stick a couple of spare peas into the ground before I leave in November, perhaps with some sort of cloche over the top, just to see what, if anything, happens to them.

Yes, I shall find room for some sweet peas in the ground next year, tho I do like the height the pots give to the courtyard. I've just discovered there is one called Jilly which is very close to my name. Must get some of those....

Can I start them straight in the ground as well then?

According to my book you can, but I think I'd be inclined to start them in a pot and then plant them out. Check the packet though, because some of the seeds need to be soaked or scored a bit if they have a hard coating.

Yes, you can do the same as recommended by Bamboo. I grew some sweetpeas over winter on my balcony in small pots with a minimum of protection. In the spring I planted them where I wanted them to grow & they flowered lovely, in fact one plant is still flowering at this moment!