Apparently, the answer is simple and quite unsurprising:

JK Rowling's chemistry teacher.
-
From [this article](http://www.southwalesargus.co.uk/news/1559519.jk_rowlings_inspirational_teacher/) about John Nettleship, who died aged 71:

> "The first I knew was when a reporter from a national newspaper knocked on the door seven years ago and said: 'You're Professor Snape aren't you'," said the former head of science.

> "I suppose I was quite strict as a teacher, but I said to my wife, 'she thinks I'm Professor Snape'. She said 'of course you are, but I didn't want to tell you'.

> "Fortunately for me, quite a lot of people like Alan Rickman, who plays Snape in the films."

And he does look a bit like Alan Rickman, minus the curtain of dark hair:

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/h0Inm.jpg