On the systems with control panel, When you arm the system, (enter pin and press OK), you should get two options, Arm and Home. If you select Home then any PIR that you installed correctly for Home mode will not activate the Alarm.
To set up a PIR for Home mode you need to program it to "home Omit" mode, instructions under "Further PIR Programming" under "Configuring your system"
If you are still having problems, Call their helpline on +44 1902 364606, I could not figure out something relating to the keyfob and I found them very helpful and they sorted my problem.