Thanks for the kind compliments, Both. Despite a logical approach though there were a few hours and some cussing involved in the final step before the penny dropped!
To answer your question, Simon, the phone was in "charge only" mode since it was connected to the powered up PC with the keypad locked, and it remain locked thus until some hours later the PC was booted. I will however try unlocking the keypad and then restarting the PC before the keypad has locked again (I'll temporarily set a five minutes keypad time out to play very safe) to see if "inverse logic" happens!
For completeness of interim info the two top panel USB sockets on the PC stay live even when the PC is shut down. Only if I flip the power switch on the PSU itself do they lose power. It is for this reason I have got in to the habit of using them for charging.
Stay tuned for the feedback promised above,
XV