I have been offered a W reg 1.4 polo which is in really good condition, but both speedo and rev dials etc have stopped working. I have been told that if the sensor goes into the gear box its a simple fix or if it is an ABS sensor it will be expensive and a big job. Please can anyone confirm where the sensor is in this model and what the cost might be roughly from a local mechanic??

The fact both the speedo and the tacho have stopped working indicates a fault common to both and that is unlikely to be the sensors you mentioned. The tacho obtains the necessary signal from the engine management and not a speed sensor.

The problem could be with the supply or ground connections to the instrument panel, the actual supply or ground or perhaps a faulty printed circuit...

The speedo is controlled by the VSS sensor in the box and it is not all that expensive
The rev counter ( tacho) is controlled by the ECM as it get is impulses from the ignition control module
nothing to do with the ABS ( anti - brake skid)
unit which is connected to the brakes
transmission problems you take to an accredited auto transmission specialist