Alpha,
You need to tell them that if they're looking at changing airlines in LGW (between BA shuttle and Monarch), they need to change terminals and the little mono-rail that connects North and South is out of action from October (IIRC) until well into 2010. There will be busses, but people are advised that this can take 20 minutes between terminals. So add that to their connection (as well as picking up luggage and checking in again for Monarch - unless they can do that in MAN)
Gg