What the hell is a P1 or an M1?? I listen to both 105.3 and 94.9 in the mornings and still don't know why they have those dumb nicknames for their listeners.
A P1 is the industry's name for a primary listener. (in other words, for that listener, the station is their primary station, the one they listen to the most and punch into first) You try not to piss them off because they give you the most listening.
P2's are secondaries. You try to get them to come back more often and stay longer.
Both are important, because in PPM both help you. (In diaries, the P1 would just write down their fave for 4 hours straight.) The reality is that people punch in and out all the time for any given reason (they walk out of the room, they get out of the car, they don't like something, you're in commercial) so you're always trying to give people a reason to come back and stay longer the next time.
I'm not familiar with M1, other than being it the big motorway in the UK.