So I have been doing some digging. When I select "Go to definition" on Email.SendEmail, this was pointing to:
D:\Documents and Settings\Administrator\Local Settings\Temp\3564$MyClassLibrary.dll$2.0.50727\MyClassLibrary.Email.cs
This contained ONE function prototype - the four parameter function was not there - as i expected. So, I thought I would just delete the Temp folder, problem solved. Wrong. When I built the project, it RE-CREATED the above folder and file with the same contents as before. 
I then decided to go digging in registry:
I found entries containing "MyClassLibrary" in
HKEY_CURRENT_USER/Software/Microsoft/Search Assistant/ACMru/5603
Also found entries containing "Email.SendEmail" in
.../Microsoft/VisualStudio/8.0/Find, but I doubt these relate to the main problem.
I need a solution to this problem fast; thanks in advance if you can help
k