A couple of things:
I noticed the coolant level was a bit low so I topped it up. The next time I was stuck in a traffic jam, steam could be clearly be seen coming from the gap between the bonnet and the offside wing. This happened a few times and then stopped. When I next checked the coolant, it was low again. When I topped it up, I had the steam occuring again. The cooland is never that low, it only takes a very small amount, maybe a pint, to bring it back between the Min and Max markers. The dashboard never indicates an overheat or even a slight raise in water temperature.
Also, and I notice this whether there's steam or not I get a funny smell entering the car and I can smell it on the outside as well. The last time I experienced a smell like this was when the oil feed pipe to the turbo on my Dedra sprung a leak and was slowly dripping oil onto the hot turbo. I can't see any obvious oil leaks on the engine block, near the manifold or the turbo, just the usual pissy PAS fluid which is nowhere near the engine really. A rock has recently gone through the grille and damaged the aircon radiator and there's now a sheen of aircon oil over that but I can't imagine that this would really cause the smell?