Hi Olly,
After talking this through in great depth with you and Mike on the telephone this evening when you informed me that both the fans were also running slow, as you know I came to the conclusion that your over temperature issues are being caused by an earthing issue, probably on the grey male block connector which fits into the black female connector at the fans or if not there at the block connectors underneath the radiator.
Rectifying the earthing issues should solve your over temperature problems and at the same time save you £800 - £1000 which Nissan were going to have off you.
Let me know how you get on.............