I've never actually done a connection at T5 but from what I've seen, and the general lay out of the terminal I can't see an hour being a problem at all.
However, I think you'll find there is an earlier flight, I assume you are looking at the 08:20 from MAN arriving 09:25 connecting to the 10:30 BOM flight. There is a 07:00 flight option from MAN which arrives at 08:10 giving you a very comfortable 2 hours 20 to connect.
Alterntively if you risk the 08:20 the worst that will happen is an 11 hour wait in the lounges for the 21:30 to BOM (assuming you are CW or EC Silver+ of course!)