>>Two other, rather more obscure, possible causes I came across:
>>(a) Writing to any file in the bin directory (or any sub-folder thereof)
>>(b) Anti-virus software changing the datestamp of, for example, web.config......
>
>There is nothing under the Bin directory. It only contains Framework.dll and UniversalThread.dll which have been sent since a while. Web.config dates from July 2010.

Well I said they were pretty obscure - but at least you've ruled them out. Don't know what else to suggest at the moment but pls let me know if you make any progress...