It seems I have two options -
1) wait - connectivity reestablishes in about half an hour. During this time I have coverage but nothing works, no data, no SMS, no calls, in or out.
2) flick phone into flight mode and out again. This reestablishes connectivity straight away but my god what a pain.
First thing in the morning (7am), no problems. And by 5pm, things start to improve. This makes me suspect poor backhaull. (I'm not a cellular expert)
I'm a heavy data user who buys the 12gb pack nearly every other month, and I share the data with 5 other 2degrees customers.
This is not a device issue - it plagues me on both a S4 and an S5, and anecdotally several of my friends also have the same issues.
I love the cheap shareable data and makes jumping ship a really hard thing to contemplate, given the other options are rubbish, but every day I get closer to doing it. If I do switch, I'll likely take those I share data with with me.