Just wanted to post a cautionary tale as advice to others about a) changing batteries; and b) dealers' "sir needs a new exhaust" trick.

Left car (2005 2.0T) parked for a week. Battery was completely flat (I knew it was on its last legs, so more fool me for not swapping it out earlier). Bump started. Engine ran perfectly at speed/revs but stalled from idle/low revs. Had to wait 5-10mins to restart it each time. Only the ESP light at this point.

New battery. Symptoms actually got worse - would crank, fire for 1sec then die. If left for 10 mins it would start once, run for 5 mins, then die.

Audi dealer's computer apparently said "new cat". Would have been getting on for £1000 inc labour. I declined. As I drove off pondering my next step, the car, very mysteriously, now drove perfectly with no warning lights. Next day, still perfect apart from the engine light came on mid-trip but still purring along. Back to local mechanic who cleared it - fault showed as a lambda sensor.

Conclusion - either the ECU is still recalibrating from the battery change (and all in the garden is rosy) OR (I suspect) a lambda sensor is actually duff (<£100). I am not impressed, to say the least, that Audi wanted to flog me getting on for £1000 worth of shiny new exhaust and 3 lambda sensors ! If the engine light comes back on I'll just get an emissions test to confirm and then a new lambda sensor.

Lesson #1 Don't wait until your battery goes flat before forking out for a new one.
Lesson #2 Give the car a few days to readjust
Lesson #3 Don't take advice from Audi dealers at face value (see how tactfully I put that !)
Less on #4 Use a 'Battery Saver' gadget to power the ECU while you swap batteries