Hi Simon,
I had this fault on a 2006 Jetta TDI,brake lights staying on,no effect on performance,on live data switch was working ok.

The problem with this,lay in the engine ECU.Removed ECU and there was burn damage internally.

I got a local Technician to repair,and it has been ok since,this was 3 years ago.

May not be your problem but thought it might be worth mentioning,
Regards Christy Doyle.

Forgot to mention there was a code for brake light switch retrieved from engine ECU.also the hall sensor on master cylinder had already been replaced.