+1  A: 

If the control is hosted on a different machine you need to use that method to debug. See this blog post.

RossFabricant
Thanks for you help, however, all the files are hosted on the same machine as Visual Studio and IE. I have updated my question to include this information.
Ian Ringrose
A: 
  • I uninstalled VS 2008
  • I then did a clean install of VS 2008 and it did not work
  • I then decided to install VS 2008 SP1 so that I could get on with some other work
  • The install of SP1 needed a reboot (the uninstall / install of VS 2008 did not need a reboot)
  • The script debugger started to work.

I still don't know what caused the problem and lost over a day's work with it. How long before the script debugger stops working again..

Ian Ringrose