Hi Adam
See: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/939218
I wish I'd looked at this properly before!
This fixed my problem:
1) Create a file at /etc/modprobe.d/intel_11n_disable.conf containing this one line: options iwlwifi 11n_disable=1
2) Run this command: sudo update-initramfs -u
3) Reboot
Thanks to everyone who helped! Especially Adam and Jude. I haven't tried the 3G card again yet....