Emission Control System warning

[Norretal]

I've looked in various sections and can't find any reference so hopefully this is the right section to post in.

Last night I drove the S8 for the first time in five days and noticed it was "lumpy" when moving off from standstill and the warning lamp was flashing. I pulled over and turned it off for 30 seconds or so, restarted and the lamp stayed off and the car drove fine.

Came out early this morning to use it and although the car drove fine the lamp stayed on constantly. I drove steadily for the two miles of my journey and parked up, tried again a few hours later after my visit was over and same thing occurred, the lamp was lit constantly but the car drove smoothly.

The manual states to take it to a qualified workshop as soon as possible, which I will do of course but I'm wondering what sort of thing would have caused it to be activated. Has the short period of lumpy running yesterday caused the fault which now needs clearing, or is this an indication of something more serious? I'm aware of the potential damage to the cat(s) as indicated by the manual so will only take it the three miles to the garage, or is this a case where I should leave the car where it is and ask someone with the correct diagnostics to come to me?

Any advice would be much appreciated

Ron