Thanks for all the help - I now have some more info...
...out of desperation I changed the receiver (from a Spektrum AR8000 to a Spektrum AR6001e) and when I bound the new RX it worked! So I thought I had fixed it... but the APM2 wouldn't arm afterwards, but it does if I rebind each time! Without rebinding, the APM2 arms about every 5th or 6th time of connecting the battery. Really strange. Normally when I connect the battery the APM2 sits there with just the blue (C) light on solid. No arm. But after a few attempts (disconnecting and then reconnecting the battery) the blue and red lights (A & C) flash alternatively and I can arm with full right yaw. And it flies! (A bit skittish - got to play with PIDs now.) Has anyone any ideas about this wierd arming behaviour? Not a show-stopper anymore but a tad inconveinient...
Dave J: Just press the reset button on APM 2 after you plug in the battery. That should solve it. We'll remove that in the next version of the code.
Good gracious - it works! Thanks Chris. I thought when you pressed the reset you emptied the firmware.
When you say the next version, do you mean of Arducopter (now at v 2.5.3)?
When the APM Planner version changes, do I need to do anything to benefit from the changes i.e. reload the firmware in the APM?
Yes, we'll try to get that updated in 2.5.4 or 2.5.5. You don't need to do anything with the MP version changes, but when the flight software changes you can choose to upgrade (by clicking on the relevant copter icon) if you want.