Bluetooth keyboard not working - can't get past login screen!

on Leopard I didn't have a login screen, so the mac would startup without issues. I disabled the bluetooth most of the time as didn't need the keyboard to run the mac mini as a media hub.
After upgrading to SL, a login screen has now appeared, and since the bluetooth is disabled, I can't enter my password, so can't get in to enable the bluetooth!

My Logitech mouse works just fine, so are there any ways of getting a virtual keyboard on screen at login prompt? or any other ideas? last resort is to borrow a USB keyboard, but surely there's a way around this?!