Thanks for the response :) I am happy, er ok, er resigned to staying with Steam for everything.
I know in this case I could go via standalone as the F-15 is transferrable so all good.
It was more just a question of timing for this one campaign as other 3rd party campaigns that were finished after this one have already appeared on Steam and I couldn't think of a patch or other issue that was preventing it from appearing.
Perhaps I can look forward to it over the Christmas period when I will no doubt be playing it on 2.5 !