Thanks DLgrl71.

So it seems that it's totally up to the computer! When it releases the seats, it's pot-luck.

To be honest, I don't think Delta code-shares with anyone on the flights that I was talking about. LGW - ATL; MAN - JFK and DUB / SNN - JFK.

I suppose I should always ask for it at the time of reservation just in case and if there's nothing, I'll ask at check-in just in case they've been blocked off 'til day of departure.

Thanks again for your help.