I've had this a few times and need to investigate the fan etc as she gets hot quickly when sat, but this has been happening for a while whenever I get in to this sort of hot situation:

Excuse shit pic, garage was dark, had to use crappy phone flash.
When I pulled up, there was some steam coming from the front road, looked to be from the rad cap, not billowing but enough to condensate up my clocks and screen. This hasn't happened before, even when I've pulled over in the red to stop it overheating.
Leaky rad cap? Coolant hose degrading where it meets top rad? Rad leak?
Maybe I should jump on that Samco hose group buy!