Thank you for your reply.
I deleted and reinstalled quartus pro 18.1 and then deleted the qdp folder and recompiled. I can now used Signaltap.
Thank you very much for your help in resolving this issue
For future readers that have this problem try the following:
1) ensure that you have a basic timequest SDC file: https://people.ece.cornell.edu/land/courses/ece5760/Quartus/TimeQuest_sdc_file.html
2) recompile with the new SDC file added to the project and ensure that there are no timing issues (especially with signal tap clocks)
3) If the above does not work, close quartus and the project, delete the 'qdp' folder. then re-open and recompile.
4) if this still does not work, uninstall and reinstall quartus (not as messy as you might think), remove the qdp folder, then re-compile.
I hope this helps future users.
Best Regards,
James