A quick update on this. Most important things first, I've been into the 3 store twice, and have had OK customer service both times.
My 'phone went back for repair (they forgot to 'phone me when it got back to the shop, so that added an extra week onto the 3 week wait). It date and time synced the first time when I put the SIM back in and switched it on. But experimenting with it it wouldn't sync the time in any way if I reset the time to something deliberately incorrect. I tried more or less everything starting from taking the SIM out etc.
The repair sheet said they had reflashed the 'phone (which I thought they weren't able to do on account of ZTE not releasing an eprom), and exchanged a component in the 'phone (requiring soldering) - the technician I spoke to on the 'phone though said not to take too much notice of what the repair sheet says, they have to put certain things down for certain reasons or something, so not sure what to make of the whole thing really at this point.
Anyway the 'phone is back in for repair a second time, this time by post. They did though do their upmost to convince me that perhaps these 'phones did not sync the time for you automatically. However if you tick the box that says 'automatic' under Date & Time settings, and it deactivates all the manual options (including the timezone setting), then it really should set things automatically. I wouldn't like to sell it on without this working.