Ok, the good news: it now works! Not sure what I did wrong the first time, but restarting the bluetooth daemon and the keyboard and scanning with the bluetooth manager GUI as I belted dementedly on CTRL-Fn-Fn got it recognised.

Piggy-backing here ... I have just acquired a Trust 18187 4.0 Bluetooth USB dongle, installed various bluetooth packages (bluetooth, bluez-utils, blueman) on Raspbian and can successfully use the blueman scan to 'see' the presence of my iNoPhone 4S. I also have a Think Outside Stowaway bluetooth key...