i've seen 5 cars around the same year with
exactly the same fault and its been the brake pressure sender inside the ABS pump, which requires a new abs pump.
Sorry i'll explain a little more in-depth.
The several vehicles that i've seen have expecienced these symptoms:
-TCS or ESP light, out of the blue, comes on perminently
-It cannot be turned off with the button on the centre console
-when the fault memory is read it shows a fault with the brake pressure sensor
-the fault is fixed and not sporadic
The fault doesnt effect the ABS and doesnt bring on the ABS light, only the TSC/ESP. Normal braking is not effected. It tends to effect vehicles from 55reg to 06reg (only new leon, toledo and alteas)
The only way to rectify the fault is to replace the pump (hydrolic unit) because the sensor is intergrated. Once the new pump is fitted the sensors which the ABS and TCS/ESP use must be re-calibrated.
There doenst tend to be much that goes wrong with the TCS/ESP on new leons, toledos and alteas, its one of the systems VAG have made a good job of designing.