Thank you for your reply.
When this happens I have checked in Manage Connections and mobile network is shown as 'on'. I have tried switching off then on again and it doesn't solve the problem. The mobile network status displayed is 'Orange', 'No Service' and the signal strength is shown as '0'. If I pull the battery and reboot the phone it solves the problem straight away, but it's not really practical to keep checking the phone and rebooting it all the time.
As this has been happening since I got the phone and before I loaded any additional software I don't think it can be this.
Does this sound like a fault with the phone itself?