I have to use the P13 from time to time - never in the morning or at peak time. My general tactic is to arrive at least five minutes before the next one is due, but be prepared to wait up to 30 minutes (which has happened on a number of occasions) from that point - despite the bus frequency being supposedly around 20m. I can forward you a selection of increasingly angry text messages, cursing the very name 'P13', sent to my girlfriend as I wait, if you like.