They now offer a £5 every 30 day bundle. Which gives you

250 minutes to 01/02/03 and UK Mobile Numbers.

3000 texts and 500mb of data

http://mobile.postoffice.co.uk/call-rates/bundles/

Currently you can also Get FREE credit with Double Top-Up

Double top-up gives you free bonus credit with the first top-up you make in every 30 day period. This free credit can be used towards standard UK calls, UK texts and UK data. To get the extra £5 on the £5 top up,you need to top up in a Post Office .

After 30 days any un-used free credit expires, but the next time you pay to top-up you’ll get another free credit to use over the next 30 days, and so on

http://mobile.postoffice.co.uk/call-rates/promotion/

Runs over the EE Network. 3g or 2g only.

Probaly the  cheapest deal on that size bundle around at the moment.