NSK, based on the top center and top right strikes, I can see that as a possibility. But in the most complete strike it doesn't look like an "S" – to me, anyway. Amersham "feels" too wide to be it, as even my initial guess at WAREHAM seems to be.
No guesses above were found in the British cancel database or the Stampboards thread that feeds into it:http://philatelicweb.com/pmks/
Just for laughs, I went through current and completed eBay
listings, but no joy. Interesting to me, though.