Ok, so l’ve finally sussed it in “Arming modes”. Part set had originally been set to instant as there were no devices that triggered en-route from the keypad to the upstairs. Now that l have a device (PIR) there, there is.
Ive set the Part Set arming mode to Timed and all is well, well nearly.....
The Exit time is set to 30 seconds to allow time on Full Set to walk to the front door, grab my keys, open/close door etc.
When Part Setting l like to hear the setting chime so l know it’s done, however, that now takes 30 seconds, by which time lm in my bedroom.
So, is there a way to have two timers? I.e. Full Set left at 30 seconds, Part Set at 10 seconds?