flights with 1h20 time between arrival and departure?
And that assumes the inbound departs on time and then doesn't sit around in the hold.... you could easily loose 30-40 minutes due to inbound delays (at any airport, not just MAN). Then you assume the baggage comes off the belt in time (at any airport, not just MAN).
Personally, anywhere in the world, I would say 2 hours... or 1.5 if you want to push it. But anything less than one and a half and you're sort of asking for trouble.