Internet problems on three (3) network UK

Does anyone else use a factory unlocked iPhone 4 on the "three network" in the UK?

I recently switched from O2 onto "the one plan" for all-you-can-eat data but am having problems accessing the internet sometimes.

It seems to be when I'm at my desk at work, although I have 4 bars of signal and it is showing a 3G symbol.

I'm running JB iOS 4.3.1.

The APN is automatically set by the SIM and isn't normally editable on this network, however I have activated the option in the menu (edited the Plist file) and have confirmed that the APN settings are correct.

Anyone else having similar experiences? Any suggestions?

Methods tried to date:
1. Replaced the SIM card
2. Reset network settings
3. Numerous reboots
4. Activating and deactiviating Airplane mode
5. Created a custom network settings profile using the Apple iPhone Config Utility

The customer services are script monkeys and have no idea what I'm on about.

Bump bump

Just got off the phone to three uk's technical help and they have told me to turn off the wifi toggle. I'm not sure what difference this makes as I'm not currently connected to a wifi network where I'm having the problems anyway.

But I'll give it a shot, then phone them back when it doesn't work.