Since posting the this topic, i found the following
This issue seems more to be with Dreamweaver and templates rather than any of the extensions ie (siteassist)
If i create a blank page (no template) and insert a table with say many cols, when I add content into the cells if the table extends past the end of Design WIndow on the right hand side, I get a scroll bar on the bottom of the page and can use that to scroll to the right.
However If i build a page based on a template and perform the same task ie build a large width table, the issue arises where I cant Edit. (no scroll bar appears) As an example if while dreamweaver is in full screen mode, (i have a widescreen monitor) I can build the table to the end of the editable region (which is the displayed page (designer window). If I then reduce the size of Dreamweaver ie (half the monitor width) which would make the design window smaller, the table becomes un-editable in the design window. I have added two more screen shots hopefully to illustrate this. The images show the same table in a template page the goodpage was taken with Dreamweaver in full screen mode, the failedimage shows when I reduce the size of dreamweaver and thus the size of the design window. As soon as the design window is smaller than the width of the table the issue arises where I can edit inside the Design window.
So what I had to do with the pages that I created with SiteAssist and then DA, once the forms were built, If I wanted to edit the page, I had to remove (cut ) the table entries with the data rows
ie cut from the form table, the data rows such as:-
<td class="WADAResultsTableCell"><?php echo($row_WADAvw_team_results['Round']); ?></td>
<td class="WADAResultsTableCell"><?php echo date("d/m/Y", strtotime($row_WADAvw_team_results['cr_date'])); ?></td>
from the page. This would then make the table appear small and entirely within the Design Window. I would then be able to edit the page, e.g. say add some extra text etc. As soon as I was finished with the edits, I would copy back the table data rows back into the page (which would then stop me being able to edit the page), save it and upload it to my site.
So if anyone out there is having issues with the design window not being able to directly edit then look at the content of the page and to see if that content extends past the width of the Design Window, you may have to cut and paste if you want to make edit changes.