Seems like a factory reset hasn't solved it either. I've just tried taking the SIM card out and restarting and the option for 4g showed up again in the network mode screen. For some reason, it disappears again when I put the SIM back in. It seems odd that I had the option there before the update, but now my SIM seems to be blocking me from accessing 4G.
I only started the contract over Christmas, so I should have been given a 4G SIM, and it wasn't blocked before the update, which should have been unbranded anyway.
Any more thoughts anyone, because I'm stumped.