Hi,
Just an update, after further investigation the noise was found to be coming from the rear left wheel and only noticeable during slow speed driving. It turns out the valve sensor had broken off and was rattling around, so big relief it wasn't something more serious! Looks like I need to get a new one as it's badly damaged.
If anyone has any spares of these knocking about i'd gladly buy one off you (same one as this: http://www.ebay.co.uk/itm/Nissan-Infiniti-40700-1AY0A-Tire-Pressure-Monitor-Sensor-OEM-FREE-SHIP-/221938488865)
Cheers guys,
Tom.