Set child poll timeout in co-ordinator XBee Gateway

I am trying to set a child poll timeout of an hour so if I have set SN=60 and SP=2000 I believe this should give me a timeout of 3 x 20sec x 60 = 3600 secs ie one hour. To test this I associate an end device and observe NC getting decremented (20->19). I then turn off the end device and monitor NC. After 250 seconds it increments back to 20. This is causing me a problem as it is forcing a lengthy re-association.
Can anyone tell me how to set the timeout to an hour?
The firmware on both the xbee modules is 405F

What is your SO set to? It should be set to 0x10.