I can only speak for the Southport line, but I'm 100% certain every Sunday during the summer they run every 15 minutes. I can see Seaforth station from the street outside my house, and I was using them yesterday; every 15 minutes they were running.
Anyone follow NHL? With Everton being rancid i started watching the odd NHL game after a mate recommended the Oilers being good to watch and I am starting to really enjoy it. Would recommend if you have never watched it.