I use Giffgaff but only for voice and text and find their deals unbeatable. Call and text relibailty has been 100% so far and for £5 a month I get 100 mins to any UK phone, free 0800 calls, free calls/SMS to other giffgiff numbers and 300 included SMS messages. I also get 1 free outgoing call minute for every 1 minute that is incoming.

The ideal secondary phone PAYG tarrif I think...