Powering a Nano with a 5v mini USB adaptor
Thanks all. I found a different power source with a mini-usb connector and it works fine. I think when plugging it in to the PC the Nano makes some faint clicking noises but when direct to  5v plug it does not? I'm not sure so I'll re-test. It was the lack of clicking that lead me to believe it wasn't working as I can't see the on board LEDs using my enclosure.

I don't know about the accuracy but it seems to work for my situation - it is to be placed in a fridge that recently turned itself in to a freezer but has since fixed itself. This project is an early warning system in case it decides to do it again.

Thanks again.
Jack