Update - one of the xenons has just went out... it flickered for a bit then went off completely. I really hope the bulb hasn't blown because of this.
The dash has all the red and yellow warning lights on (ABS, airbag, traction control, battery, engine diagnostics, seatbelt fastening sign).
I'll leave a note for them and I guess its Audi Emergency Service when they get back. Its a shame - the car is only about 2 years old and pretty high spec... I remember my neighbour telling me about it when her father bought it (A8 3.0tdi quattro).