PeterH:
I have a suspicion it may be due to internal behaviour of the RF24 library.
Well, that may very well be! 
Radio timing can be complex, or it may be a straight-up bug. Thanks for the example code, I'll try it out.
What rev of the library do you have? If you downloaded it from the "download" link, it's in the name of the zip file. If you cloned it from git, do "git log -1".
First thing I would try is a different pipe. Try to balance the 1's and 0's in the pipe. The examples from Nordic have pipes like that, e.g. A8A8E1F0C6, etc.
EDIT: Could you also try it with the radio ground tied permanently to the Nano's ground?