Timings
I have done a number of timed tests on this (the wide variation on transit times reflects the potential gaps in service). I am 40 something, possibly in better health than many people my age. I also carry my life on my back, not on wheels, I don't do checked luggage; there again I didn't have the spousal and filial units in tow. I also know Terminal 5 very well. So bearing in mind these factors you may need to adjust these timings accordingly:

From T5A South Lounges Complex, by exit to Flounge, using the lifts.
- To T5B by transit - minimum = 6 minutes, maximum = 21 minutes, reasonable average = 11 minutes.
- To T5B by passageway = 7mins 30 seconds

From T5A South Lounges
- To T5C by transit - minimum = 8 minutes, maximum 24 minutes, reasonable average = 14 minutes.
- To T5C by passageway = 10 minutes 30 seconds.

From T5B to T5C or vice versa
- by passageway = 4 minutes 30 seconds.