Just an update. I sent an email to Talkmobile and they replied that Vodafone had introduced content restrictions and that was also probably applied to proxy servers. They weren't aware that Opera Mini was affected. I was asked if I wanted the restriction to be removed. This I did ask for.
Yesterday I received another email saying that the content control and premium info bar would be removed from my account. This would take 7 days.
Today at 11am, I received a text saying that I had full access to the internet. I'd already been told to reboot the phone. This I did and since then it has been full speed ahead using Opera Mini. I've closed and reopened Mini lots of times since and no problems using 3G at all. So the solution is to contact Vodafone or your telco if they use Vodafone's network.
Only passing comment I would say is that it would have been nice to have been notified prior to parental controls (which is what it is) being introduced.