Raising the matter with Canon, I had a reply that combined the helpful with the bizarre.
We are sorry that you are having problems locating the driver on Mac OSX 10.7 for your i-SENSYS LBP5100. The driver has been created, but new drivers are not always available on the UK site, as it is currently under maintenance.
The driver is available on the Australian site, at the following link:
CAPT Printer Driver Ver.3.40 for Mac OS X 10.4/10.5/10.6/10.7
http://support-au.canon.com.au/contents/AU/EN/0100359005.html
Have not tried it yet, but it could be a useful secret.