On the way to work today the ASR light came on permanently, Switching on/off cured it and I'll VAG-COM it later but has anyone got any ideas on what could be the cause?

it can related to remapped cars with excessive torque... or also the sensor on steering column. have you had the wheel off at all? If there is a slight misalignment with steering wheel/ tracking it can confuse the ASR hence it shuts off.

The other option is dirty abs sensor on one of the front wheels.... this can be caused by failing cv boot.... not that either scenario happend to me ever.... honest.

Not had the wheel off (that is next weekends job, CC fitting).

Could be a dirty abs sensor to be honest.