iPhone 5 refuses to automatically switch to 3G

I have an iPhone 5.

Unlocked.

On a SIM only plan from Vodafone UK. In areas where I know there is 3G it will only connect to EDGE/GPRS.

Going to Settings>General>Mobile Data and sliding 3G from on to off and then on again kicks it to the 3G network which I knew was there all along.

Tried resetting network settings.

Tried gently stroking it and whispering sweet nothings in its microphone/ear but nada.

Any solutions? I can confirm this is happening on my boyfriends phone as well. Exact same setup as me.

Many thanks

Tom