iphone connected to wifi but no internet

I have an Iphone 4 with IOS6 installed and am connected via wifi but no internet. My phone has been connected to the same network for the past month with no issues and even this morning was ok but now will not connect to the internet. I had this problem previously on same phone with IOS5 but internet would come back on after a few hours.

I have tried doing the normal bits - renew lease, forget network, renew network settings etc but nothing works.