This issue has occured again - the host machine rebooted with the project open and now the file is locked again. I can neither open delete nor modify it. Whenever I try and do any of those I get the windows message "This action can't be completed because the file is open in another program. Close the file and try again" But the file is not opened. The host machine has restarted and I can't open the file. I will wait until tomorrow morning and see if it is magically available for opening then.