Hi Celliott,
No, I didn't dismiss Durance through dialogue - in fact, if you talk to him (or any of the other companions) the dialogue options suggest that he still believes his is travelling with me. I even get the dialogue option of 'let's get moving' or similar, but they do not join.
Durance was in my party and I went to an inn to try and recruit new adventurers (as all of mine were missing from stronghold which prevented me from carrying out any adventures or escorting duties). In the party management screen I had to dismiss Durance in order to add my newly created monk and level her up. Unfortunately once I'd done that, Durance was not in the stronghold and was no longer recruitable from Magran's Fork.
I have just repeated the action now by creating a new priest and dismissing everyone apart from this new character from my party - when I go back to Caed Nua, they are all sitting there comfortably. So this issue doesn't appear to be happening any more. However, when it did, it meant losing all the companions I had recruited and now I cannot get any of them back except the 4 I have in my party.
I'm guessing this means going back a month or so to saved games where everyone is still available and starting again from there? Unless you have any better suggestions?
Many thanks for looking into this.