Yes, it's going to a local Audi specialist next Tuesday, I'm sure the first thing they'll do is read the codes, and then maybe a compression test to rule out the head gasket, but as said they told me over the phone that was unlikely to be the issue. If the thermostat is faulty then yes that will be replaced, and does look like it needs another oil change too. So all depends on their assessment, I'll update this thread after they've had a look at it. Thanks for the suggestions so far.