Cannot connect to any O2 wifi hotspots

My iPhone has stopped connecting to any O2 wifi hotspots. It used to connect fine but a couple of months ago it just started coming up with the error message "unable to join network". I have tired resetting network settings, restarting the phone, turning the wifi on and off, disconnecting and reconnecting and pretty much everything else support says to do when it wont connect. Is this a problem with my phone or the actual wifi? My phone connects fine to any other wifi.

Thanks