OUR thanks to CWS Washroom Services for enabling Keeping Posted to break, as they call it, ''one of the best kept secrets in Europe''. And what might that be? Shock . . . horror . . . one out of every four toilet users in Germany, Switzerland, France and the Netherlands does not wash his/her hands after visiting the WC. The CWS people inform us that 7,679 men and women were discreetly observed in 408 washrooms - although they fail to tell us how many of their researchers were arrested during the course of their onerous duties. Of course, it came as no surprise to discover that CWS, who sent us this gloating bit of information on the toilet habits (or rather, the lack of them) of the Europeans, was based in Singapore. As we all know, in that pristine city state fondly known as Nannyland people could get locked up for life for not washing their hands after visiting the toilet. And hung and quartered for not flushing it after use.