I see that Virgin Mobile Payg have altered their terms and conditions to say they can expire a PAYG SIM if no call or text is sent for 90 days, it used to be 180 days .This could catch out infrequent users.
"3.8 Keep talking! : You need to use our Services at least once within any 90 day period either by making a chargeable call or sending a text message. If you dont we may suspend your access to the Services and cancel this Agreement. If we cancel this Agreement, you may lose your phone number and any unused Top Up credit on your account (see Clause 10.3). We dont really want to lose you as a customer so please ensure you keep talking or texting"
http://www.virginmobile.com/vm/genericContent.do?contentId=our.service.footer.sm068#Terms and conditions of service - Pay As You Go Addict and Simply 8p Tariff