There is one gearbox speed sensor on the manual box, and three on the CVT, by 'driven pulley speed sensor' I assume you have a CVT.
The driven pulley and final output speed sensors (also known as pick-up assemblies) are easy to reach once you have removed the air cleaner housing as they are on the top of the gearbox next to the linkage, the drive pulley sensor is on the side of the gearbox. Removing, cleaning the probe and checking the electrical connector is secure may fix it.
This part is common to many Honda vehicles although it has several variations depending on the mounting flange. The Jazz uses part 28810-P7W-004. New from the dealer they are around £100 plus tax, generic eBay sensors are ~£20, but you might remove one from a salvage car for £1.
http://www.hondafitjazz.com/transmission_cvt.htmlhttp://www.ebay.co.uk/itm/28810-P7W-004-New-Car-Transmission-Speed-Sensor-Fits-for-2002-2006-Honda-CRV-/182641736470Drive pulley speed sensor: 28810-P7W-004
Driven pulley speed sensor: 28810-P7W-004
CVT speed sensor: 28810-PCJ-014
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TG