Keeps stopping and starting, the mileage, is on there all the while, but the speedo petrol gauge etc goes off then comes on again, when it feels like it, it's a 2000 audi tt
Faulty connection / solenoid somewhere (genius, I know). Check that all fuses are fitted correctly as well. Do you know anyone with a diagnostic device as this should identify it?
The instruments which are erratic are live only when the ignition is on, the mileage is illuminated usually by opening the car door and remains on for a time after you leave the car. This implies the erratics are on a different circuit and they are often driven via a voltage regulator for stability reasons. There could be a problem with the voltage regulator or just a loose connection somewhere in the circuit.