"Eventually also realised that my problem was related to faulty 3.3 v regulator.  Replaced it carefully after watching the excellent tutorial by SparkysWidgets at https://www.youtube.com/watch?v=z7Tu8NXu5UA.
APM was working fine after replacement, BUT…"

"Very same problem here on APM 2.5.  Tried factory reset, previous versions of MP, ArduPlane, ArduCopter, ArduRover.  All hang at some stage of the connect stream.  Do succeed once in 20 or 30 attempts to connect.  Then mostly without initialize or…"