Only one way to find out, Paul, isn't there!
If the demijohns have cork bungs, and are presumably standing upright, there may be a problem, because as you say, temperatures vary widely in a roof space, and they could have shrunk the dryer they got. So that would mean that air would possibly have got in, and caused contamination. If they're rubber, then there could be a different result, and you could strike lucky.
Be careful opening them, as although they may have been stored when finished, there could just have been some secondary fermentation, and while demijohns don't take pressure, you could get a bit of spray...
Sounds like an intriguing project, there could just be some gems there!