Discovered what was wrong..
My website was running in a virtual directory. http://localhost/myweb/
The web.config in the virtual directory was set up correctly as was all references
in the web pages. But the web.config in the default directory http://localhost/ still referred to the old version of crystal. I had thought my application would only look at my web.config, but obviously not!!
I removed all references to crystal in that web.config.. And all worked.
That's the problem resolved.