Giffgaff, the O2 MVNO has recently revamped their PAYG monthly bundles, aka "Goodybags".
Overall, I am pleased with their new offerings, because most of the Goodybags now contain more data and minutes, which makes them much better value than they were.
No price increases this time (thankfully!), so giffgaff's Goodybags are now much more in line with many of the other networks.
For me, the £10 Goodybag is now pretty great value for PAYG, though 2GB of data is still a bit tight - SIM only deals offer more at this price.