Just in case anyone is interested, I made the trip safely enough with plenty of time to spare. My LHR-BOM touched down at about 11am and I was in the domestic terminal by 12 noon - about an hour in total! ^
To consider my specific case:
- My incoming flight was on/ slightly before schedule, docked at a gate so no bussing or anything
- Immigration took me about 5 - 10 mins, lot's of open counters not that many people
- My luggage came out in another 5-10 mins after clearing immigration
- Customs, re-scanning and checking in with the 9W transfer desk another 10mins
- The wait for the transfer coach to leave another 10mins
- Actual coach travel about 25mins
So yeah, in total about 60mins with everything running like clockwork. So I made it there with an hour (~35min to actual boarding) to spare. The main factor behind all this was prolly due to the fact that my incoming to BOM was at 11am which is most likely a non peak hour (can anyone confirm?) time for intl flights. However, I can easily envision the whole thing going wrong in a cascade manner if things start to slip. Thankfully in my case it was OK.
Still, I would avoid BOM transfers if I can in the future to avoid some of the hassle and uncertainty involved. HYD airport was such a treat – but again it’s much newer and a lot smaller.
//$.02