Doovey

If you give your local dealer a call and they can quote you.

IMHO there should be two unrelated problems with your car. I get the same error you have from time to time and the problem seems to be weak cat, which needs to be replaced (in your case, it could also be the front lambda for first free cylinders). The TRC and handbrake light should be because of other problem. I assume people with better diagnostics should be able to read that error tho. The TRC and handbrake lights do not come on just because of the P0420 ECU fault. The acceleration sensor needs to be re programmed due to the TRC learning ,it relies on the engine ECU for the running information (throttle position etc...) and at the moment it is being supplied rubbish information due to the faulty cat.

Looked into it some more. You need to replace the manifold cat and probably re-programme the accelerator sensor. Ignore my post about the sensor. I was given duff info. The TRC light is flashing because the fault is still there with the engine management. Engine management is ignoring the signal because of the "self adapting capability" of the ECU.

I would advise you to have the sensor replaced as this is the cause (hence the fault code). Cheers.