I've set this up so it should be a case of wiring it in and switching on when I travel to Italy in a fortnight.
It took a while - the Nest App knew I wasn't in Italy and that I was in range of a Nest Protect device I own. This meant I had to temporarily change the address of the house in Italy to the UK and set it up via a hotspot before manually forcing it to connect to my wifi (wifi in Italy has the same ID to make life easier). Not particularly relevant to the next question but I'm writing it down in case someone is having similar difficulties with Nest.
Anyway.... during my troubleshooting I noticed the heatlink for the 'E' has a micro-USB port.
I plugged this in to a standard power adapter and it works without batteries - see photo.
I asked Google who said it was for 'troubleshooting and backup' but the device works well in this state.
Does anyone know if it would do the heatlink undue harm to leave it powered via USB as opposed to 2 x AA's?