Re. mac mini mid 2011 - flashing grey folder and question mark - no keyboard connected? How do I reinstall the OS with no keyboard?

I want to delete my files and restore the OS to sell the mac mini. I pressed cmd and r while starting up to get to the recovery menu. I used disk utility to erase the hard drive. Then from the menu I selected the option to restore the OS (Mountain lion OS X 10.8.2). I got the blue bar on the screen showing it was doing its stuff and the time remaining. The time remaining just kept going up and up to about 300 hours, then down and then back up again, sticking on about 125 hours. I left it doing it's thing for about 24 hours and it was still stuck. So I cancelled the operation and shut down the mac mini. In the meantime I then stupidly took the batteries out of the wireless keyboard to charge them so there is no keyboard now associated with the mac mini. So now I can't obviously do the cmd and r on start up, and when I start up the mac mini I get the dreaded blank screen with flashing grey folder and question mark. I have a Genius bar apt scheduled for 04.01.13 but wandered in the meantime if anyone has any suggestions? The only think I could think of was buy a wired keyboard to try the cmd and r option (I am presuming this is the issue but don't know for certain), but this is an expensive option at £45! Any suggestions appreciated, in laymans terms please as I am not very technical.

Thank you.