I am a native speaker of British English; the Longman Pronunciation Dictionary and the Cambrighe English pronouncing dictionary deal mainly with British pronunciations, but also record American pronunciations.
Sorry, I gave you incorrect information in my last post.
In fact both dictionaries record /s/ as the primary British pronunciation, /ʃ/ as an alternative. They say it's the other way round in American English, noting /ʃ/ before /s/.
Well, I have always pronounced it as if it were sh. I have never heard it pronounced any other way. Apparently, there is a difference between American and British pronunciation. (I am American.)