As greg said in first post 'down and upstairs landing to be enabled' eg: pir top and bottom, pir in mid landing.
Scenario no rkp at top of landing, you wake up half dazed, walk down stairs, alarm starts entry timer, noise from rkp downstairs wakes up other sleepers, you don't make rkp in time to enter code, full activation, neigbours also woken-up, kids, wife, husband, neighbours pi**ed off
Yes you can get away with no rkp upstairs, but for the sake of £30. I'd rather have the convenience!
An electrcian will sell you any old system he can get from the leci wholesalers.
A security engineer will decide what's in the customers best interest!