Had apparently strange goings on with IIS & Asp.Net.
Delivered new mylibrary.dll. User renamed mylibrary.dll to "old mylibrary.dll", copied in new mylibrary.dll. Got error saying 'old mylibrary.dll' could not be loaded into manifest.
After deleting the 'old mylibrary.dll' files the web site loaded ok.
After a discussion we came to the conclusion that IIS loads all the DLL files in the BIN directory irrespective of references within Web.Config.
The site then throws exception when two dlls contain the same Assembly-name - this also makes sense.
So what appeared to be strange behaviour came down to not thinking logically!
