I received (via my business manager) a response from the Business Support team on the 27th October:
This is a known problem area where we have received other customer complaints over the last 6-8 weeks. We are implementing some upgrades to the 2G network this weekend which we expect to make a big difference but the customer will have to switch 3G off on his phone to benefit. For 3G we do not have any dates at this time. Please can you see whether the customer would accept switching 3G off as an acceptable workaround for the time being
Okay. So 'We know there's an issue, but hey - why not downgrade to last century's technology so your phone works. We wont give you a discount obviously, but hey, that expensive smartphone your paying a premium for will actually work as a phone. Meanwhile, we'll still keep trousering the money'.
Ah. No. Not good enough. Way not good enough.