Hi Cathi - sorry to bother you again!
After scouring the internet I discovered that this error is normally caused when the pages can't locate the .dll
I have my root folder (has login form and general forms etc) with various other folders one of which is a folder that contains all the secure forms etc including my startup.aspx page and also a webconfig etc. This "secure" folder had no bin directory so I copied the bin directory from the root folder to the secure folder and after a few errors and re-builds my startup.aspx works fine! But if I switch to
<deny users="?" />
the login page appears and I am back to where I started!
Is it normal to have to copy the bin directory into subsequent folders you create on the site? Or am I not setting things out right!