Tried something else...
I went back to the site server settings and ticked the "testing" as well as the "remote" checkbox. Then opened the SecurityAssist Wizard again. I think I got a bit further this time.
Now I get a 404 error, saying either there's no testing server (but there is) or, that the testing server specified for the site does not map to the .........MMHTTPDB.php URL.
I can see that the _mmServerScripts folder has landed in the /newsite/ folder so shouldn't everything be mapping OK? Or does it need to be in the root folder? Even when I place it in the root folder I get the same error.
I have my root folder in DW as /newsite - could this be the problem?