UPDATE
I got a call back from a supervisor around 2pm, he said they were starting to work on restoring all the "expired" SIMS and that they would all be back online today. He assured me that my credit would be restored and indeed an additional credit would be added for the inconvenience. He gave me his name (Ben Adams) and said if I had any further problems to call and ask for him.
5.45pm - service restored but my credit is showing as 0.01p when I had over £29 yesterday. I'm also receiving a "Class 0 message" every 15 seconds or so saying I need at least 17p credit to use my phone and I need to top-up. I can't get rid of the annoying message which keeps beeping away.
Called customer service, asked to speak to the named supervisor, Ben Adams. Was told not possible to transfer calls. Explained the problem, he looked at file notes, said I had been told previoulsy that my service would be restored within 24 hours!!! I explained the CURRENT PROBLEM AGAIN. He put me on hold for 15 mins. He came back and said he had spoken to Ben Adams and that head office was working on restoring credit to all SIMS and that it would be completed by 9pm. I commented that was convenient as that's the same time that customer services closes so if it's not completed by then it won't be possible to call and complain. He said, "IT'S NOT MY FAULT". I explained that it was O2's fault and that as a customer service representative he should be willing to accept responsibility on behalf of his company. He said that I was being rude to him and HE HUNG UP ON ME!!!
Called again at 6.45pm, spoke to a more pleasant customer service rep, explained I would like to make a complaint about the previous person I had spoken to. Was given an address to write to. I was also told that a supervisor would listen to the call.
I emailed my complaint to Ronan Dunne, CEO and Cheryl Black, Customer Service Director and will follow up by writing to the address given for complaints.