Don't shoot the messenger.
I had the AA back out in the morning. I explained the situation(it also did it at as I slowed down at a junction on the way home). I'd noticed that the rev counter was fluctuating, rising then dropping suddenly while idling.
Anyway AA guy comes out the next day. Has a look, cannot find anything. The revs is no longer fluctuating. So he plugs his computer in and we go for a drive. It doesn't do it, I also note that the revs are slightly higher.
Basically he gave me this spiel about how the internal computer that drives the air intake, controls etc would have reset when the battery was replaced and it takes a bit of a run to 'relearn' the voltage to ensure it gets the settings accurate. It's not done it since.
True or bulltulip?