You should be check in all the way through at MAN so all you need to do at LHR is walk off one plane, hang around for a bit, keeping an eye on the screens and then head off to the next gate. The gate numbers will appear roughly one hour before departure time, and like others have said definitely do not get on the train unless your gate number is in one of the satellite buildings!
I am told that when the new T2 opens it will also have a similar underground train which will link the satellite buildings (one of which is already in use as gates 31-47 in T1, but at the moment involves a long walk!). Perhaps that will be linked up to the T5 system in time...although at the moment you do also have the Underground which links the terminals in only 3 stops as well as the Heathrow Express and Connect services so there is probably not much room for another train service under Heathrow, although one which serves airside and transit passengers would be nice!