Right I've added a script that pings router ever 10 seconds, disabled power management in interfaces and also in /etc/modprobe.d/8192cu.conf.
Still getting network disconnected after about 20-30 seconds of no activity.
Have also tried a different wifi adapter based on the rt2800usb driver still no joy at all.
Also, tried on Pi Zero and get exactly the same (clean Rasbian image) with both adapters!
Oddly, once the network stops responding (no ping, ssh stops responding), if I unplug the wifi dongle and plug it back in, after a few seconds it comes back on.
Any more ideas?