Session library not garbage collecting

Thanks for that, super-helpful response, and I think we have found the problem!

Code:

session.gc_probability = 0
session.gc_divisor = 0
session.gc_maxlifetime = 1440

Although from having done a bit more reading, I think the CRON task will achieve essentially thing but be potentially less expensive so I'm minded to leave it as it for now.  Opinions to the contrary welcome!