Driving along the M25 in stop start traffic, when the temp gauge goes right up accompanied by the red light. A few seconds after, white smoke (steam?) starts coming out from under the bonnet of the car. Then i smelt what i can only describe as a smell similar to celery, coming from the heater. Pulled over on hard shoulder. When I got out the car, there was a leak, an orangey fluid (Antifreeze?). Called the RAC, the fella did a cracking job. Turns out the thermostat is bust, engine overheated, had below minimum water/antifreeze. Drove home and got a wiff of the same chemical smell again on for a few seconds. Any ideas? (Blockage of some sort?)

did you continue driving when red light came on?

is it overheating still?

The car overheated and blew out coolant. If you're very lucky no lasting damage has been done. Did the RAC man replace the thermostat on the roadside?
The escaping coolant would have been drawn into the cabin air intake, hence the smell and after the repair, whatever he did, there would still have been more coolant lying about some of which would have been drawn into the cabin as it dried up.