Quite simply phone your boss up, and simply say I understand pay day is meant to be today, my pay has not be paid, please can check why, and tell me when I will be paid as my landlord is waiting for his rent, and I have other bills to pay, hopefully this will make them sort it for you, if not phone up Acas, as suggested by frenchielove they will speak to your employer, and take necessary steps to make sure you are paid, your employer is breaking employment laws and its contract with you which is another violation of workers rights if they do not pay you, or pay you on time without a good reason ie breakdown of pay system from them too bank, or an error like that is acceptable simply forgetting or thinking they can pay you when they want is not.
