Hi cotty, only started seed swopping this year, mainly on here, but I did go to one locally and that was in March. It was a very relaxed swop so much so that I forgot to take any seeds, and just gave a donation for any seeds that I wanted.
The one I went to was advertised in our "green book", maybe check with the council and other groups to see what's out there already or as a way to advertise.
Think the hardy plant society has a yearly one which might be useful for information.
Good luck with yours, I've really enjoyed all the swopping I've done so far