It would seem that when I recently upgraded the DS environemnt the Boot environment build choices were reset to the default which is Automation folder only. I should have looked at this earlier...
Once I had asked it to build all the types of Boot environment the problem was solve.
incidentally quite how the random PXE server got into the database is not clear, but as the Guid was not in the item table deleting the record from the sbs_pxe_Server table seemed to fix it and hasn;t affected anything... I think!
Am now trying to stop w3wp.exe from killing my demo system environment!