Many thanks for your continued patience. I have completely uninstalled and re-installed Validation Toolkit, as you suggested and the program worked the first time right up to putting the code onto the page. Then in threw up an error message and didn't do anything. I've attached the message.
I'd be grateful if anyone from WebAssist would be able to assist with this problem as it doesn't appear to be caused by any other issues: I have disabled all other extensions in DW, including all other WA extensions and still get the problem.
I'm running DW8 on XP SP3 with Validation Toolkit 2.3.5 as part of the Super Suite.