Thank the lord for internet forums! After leaving connected to usb port overnight - eagerly woke to find...nothing. Left charging in usb 2.0 port all day whilst at work, eagerly checked when came back home...nothing. Symptoms at this point were the same:
1. Holding down any combination of button did not work - most common sight was the battery with volt symbol & red line, followed by plug symbol.
2. Reconnecting to laptop netted the battery/volt/red line symbol, followed by use the correct charger screen.
3. Connecting to the mains charger - resulted in nothing.
Then the miracle occurred, the sequence of events being (in the space of 2 hours) although most steps are superfluous - i've kept them in to show the method (however mad it seems)
1. Disconnected from usb port to plug into mains - no change
2. All combinations of button presses - no change
3. Reconnect back to usb port - no change
4. Disconnect & reconnect to mains charger - continuous battery/volt/red line symbol
5. Left on this screen (because it was something i hadn't seen)
6. After 20 -30 mins in booted to moboot (which i was really lucky to catch with 2 seconds to go until ICS boot) when i put into WebOS and now charging.
Upshot is that i think the trickle charge via usb for circa 24 hours followed by quick top up at mains got it enough charge to reboot.
Hope this rambling helps anyone else suffering the pain!