A new day and fresh mind works wonders.
I deleted the autosave.xml file again and restarted DW CC. I then set up a new site definition and proceeded to create several DataAssist pages using the built in presets. All with no difficulty. tables created as well.
I then (Having deleted all previously created files so a new site basically) created an admin area using the preset that I had created when it all went wrong. Again I got the error messages. So I attempted to delete the preset in the preset manager but it wasn't listed. I found the file in the configeration folder and deleted it there.
I then started again. A deleted the autosave.xml file and restarted DW. I then created the datassist pages from scratch and save the preset again. This time all the pages were created successfully with no errors.
On the original site I was updating a previous webassist set of admin pages made several months ago with a previous version. I can only assume that there was a conflict in there somewhere.
There was one thing though:
I has two tables in the database. the second one was to use as a dop down menu to select a category. When I went to the configure field settings dialogue page abd set the category to "menu". When I clicked on the lightning bolt it didn't pick up any tables "No tables found".
No problem as I'm proficient enough t add the funtionality by hand but.... just thought I'd flag it up.
Anyway I'm happy.