I opened a new account over a week ago and got a confirmation that my order is on its way the following day. I received a letter confirming my Direct Debit has been set up (so address is correct). Seven days later and the SIM hasn't arrived. I have a PAC from my current provider and this will expire soon. How can anything take 7 days to arrive within the UK? Royal Mail say that second class mail takes 2-3 working days to arrive. This means you haven't actually sent it.
