Hmmm, a couple of thoughts....
My alternator light comes on occasionally and did so quite often when I got the car, initially I thought it was down to a dodgy connection within the loom at one of the connector blocks which is strapped to the diagonal brace below the inlet manifold (mine is a 2006 K series) as when I wiggled the block the light would often go out.
However, since having purchased an Oxford Oximiser 900 and used it religiously the light rarely comes on now, only when I've been using the battery at bit for lights, so I'm thinking that perhaps the battery is not in great shape, or the alternator is charging only just enough.
In terms of heat lagging for the exhaust primaries I was considering this, but having read various threads on Blatchat I decided against it - apparently the K series doesn't cope very well with the additional heat soak from the exhaust pipes which would normally just radiate away but is trapped by the lagging.
All the best
Richard