I almost made that mistake at Sandown once. The only thing that stopped me from flinging open the bonnet, which would have sent in a massive burst of fresh oxygen and flared up the flames, was a secondary bonnet strap I forgot I had on there. So I poked the fire extinguisher through the gap towards the exhaust manifold and let her rip.. luckily the fire extinguisher did its job. It does make you wonder if the minimum required 1kg extinguisher is enough for bigger fires, but very thankful I had one ready to go! Secondary bonnet strap is a good thing.