In front of the radiator just below the bonnet latch. Search for Ambient sensor to find more info.
It's not repairable, it's a 20 cent NTC thermistor of about 22K wrapped in a plastic box that Nissan Australia charge up to $180 for, no idea what it's worth in the UK but given NTC thermistors are more widely used over there than here they are probably a dime a dozen and Nissan are still a rip off.
