An easy solution for light/irregular users:
Sign up for PAYG but pay by direct debit:
http://www.virginmobile.com/vm/gener...t.howdoi.sm087
There's no minimum monthly spend.
And calls drop to 5ppm after the first 5 minutes of each day:
http://www.virginmobile.com/vm/gener...rif.shop.sm018
And you get 'free' roaming etc.