Right the iphone 4s on the apple site says it's unlocked. So if I stick in my 02 pag & go sim will it work alright?

If you don't have a micro SIM then you'll need to get one from O2, but aside from the physical form factor of different SIM cards it will work on any UK network.

Or if you have a mini SIM card then you can trim it down to micro size yourself.

http://www.macnotes.net/files/2010/0...3g-iphone4.pdf