I don't think this is a particularly well thought out riddle.

It doesn't give you enough information to figure out who this Simon is, so you're left taking stabs int he dark until you find the anwser.

hes the pub owner

6 UK pints is equivalent to 3.40956 L (apparently) and I'm guessing Simon showed up cos Nick or Kate invited him :)

Simon Cowell makes sense. After their three pints, they were so drunk they decided to audition on Britain's got talent where Simon Cowell told them it was the most horrible audition he had ever heard.

Simon is the guy that serves the drinks, and he is there because he has been rostered on.

well after drinking they were hungry so they got a simple pie man to show up

Assuming that they drank it really fast and they are both small people, Simon is the Ambulance driver, cause they both passed out.

so... are any of us correct?

No, sadly.

The answer related to Shaun of the Dead....Nick, Kate and Simon are all in that movie, and Simon showed up because he said he would.

I don't know about 8%... but when I was living in Denmark I had some 10+ % beers.

It was called Årsøl (or something close to that), which means "beer of the year". Every year they make a new one which has 0.1% more alcohol than the previous year.