Just noticed that O2 have upped the allowances on their O2 12 Month Simplicity Tariffs as well as offering 1 Month Free if purchased in January 2010.
£20 per Month - 12 Month Contract - SIM Only
+ 1 Bolt On
1 Month Free (If purchased in January 2010)
and £23.33 Quidco / TopCashBack
Shame it's not the £100 cashback offered ~4 Months ago!