RTFM

In preparation for my imminent trip to France, I was experimenting with my new toy, to check that the expensive roaming option wasn't enabled. In doing so I managed to switch off all data network access. Trawling through the menus, I came across a "Use packet data" option, disabled. I wanted to check that this was the problem, so decided, for a change, to RTFM. I downloaded the 5.7MB pdf from the relevant website. This is the totality of its entry for that menu option:

Use packet data: Set to allow packet switched data networks for network services.

Well, gee, thanks.