I really would not buy separate tickets - especially with bmi in the mix.

Over the past couple of years bmi have being driving customers to distraction with endless changes to flight times - they've done it to me more than once on the LHR-MAN route.

If you buy a through (connecting) itinerary, then if bmi start fiddling with flight times on the LHR-MAN leg, you are covered. With a through ticket, they have to accomodate you on a suitable service

If you have separate tickets, you run the risk that what started out as a "safe" connection interval could fall by the wayside.