How do i set a alarm activation delay on my landing sensor?

I have a Accenta G3 alarm.
I live in a 3 storey house with a panel on the ground floor and one on the middle landing.
As i enter and the house i have a time delay to switch off my alarm before the bells etc sound.
I have no PIR's on the top floor but one on the middle landing. At night if i set my alarm and go upstairs , however in the morning or whenever i come back down the stairs, the landing PIR picks me up before i can switch my alarm off and i wake the neighbourhood up with the alarm activating.
How can i set a time delay on the landing PIR .
Thanks

Paul
April 2007