There are two lambda sensors on the Jazz. Although they are known as lambda sensors, they are in fact measuring oxygen in the exhaust gas.
Lambda is a ratio and at a value of 1, the combustion is at a perfect stoichiometric ratio, in other words perfect combustion. Below 1, combustion is rich, and above 1 combustion is lean.
The pre-cat sensor is used by the ECU to determine how much fuel to inject into the engines, and the post cat sensor determines if the cat is working efficiently or not.
Any half decent OBD reader will show the values from the two sensors, and normally, the pre-cat one will be oscillating between rich and lean quite rapidly, while the post cat sensor will show a much smaller variation as the cat smooths out the oxygen reading.
Bosch sensors are about £40 each and provided they unscrew OK, they should take about half an hour to change with the benefit of a two poster lift ....
£480 is having a laugh .....