Just thought I would update on diagnosis and repair. Fault was brake lights on all the time (ignition on) and glow plug light on dash flashing. Scan fault codes identified Brake Light/Cruise control circuit A.  Just replaced brake light switch using Audi genuine part £21 and cleared all fault codes. Can get cheaper pattern part but felt worth using Audi part. Fault cured and glow plug light working as normal.    A6 Avant 2007 2.0 diesel