I would echo this going from my past experience with a cat vomiting out of the blue. Its my hang up because of losing a cat to sudden kidney failure which started with vomiting. It was a totally different experience to the usual undigested food scenario, where as you say the cats bounce back straight afterwards. If Khan seems a bit sad and sorry for himself best to get him checked in case something is aggravating his tummy.
I hasten to add that my case was extremely rare but I would always seek medical attention now just to be on the safe side. I'm sure little Khan will be absolutely fine, let us know
You're just up the road from me in Stoke so thought it worth mentioning that both my 2 are a bit mopey at the moment in this muggy weather, and (apart from the obligatory mad half hours) are moving around much less and throwing themselves down on the floor!