Danny, I'm trying to keep up with your various postings on Google Maps:
Google Maps
Many of the places you name have been renamed of course, but the beauty of it is that Google still accepts the Anglicised version and plonks you down somewhere with a completely different identity. Try it. Drag yourself across the world until it gets you to Southern India. Then enter Cholaveram into the search box and it asks very politely, "Do you mean Cholavarum?", click on that and you arrive just outside Chennai, which is of course Madras. The two airfields are clearly there, just as you say. Enter Redhills Lake and we go to Puzhal Lake, close to Cholaveram all right, but to the south of it, not north. Is that right, Danny? It seems to fit the bill size wise, as it would need to with Sunderlands and Catalinas operating from it..
Ref your cryptic logbook entry, could "Air Test for 21 APx" be the name of the unit that you terrified and whose Vengeance it was? Only a guess but if it was a different outfit to yours wouldn't you identify it as such? Still no idea what it stood for of course.
Edited to add I think they were called 21 Armament Practice Camp (you were right!) as that unit was at RAF Cholavaram along with 1580 Calibration Flight, here:
Stations-C