Thanks for your response.
1) it is only occurring in PowerPoint. It is NOT occurrign on any other Office program, which print to PDF or OneNote perfectly fine.
2) I have already run the Office repair tool, to no avail.
3) In PowerPoint i can choose the Microsoft Print to PDF, or a third party PDF printer, or Print to Onenote. It promptly ignores all three and prints to my default printer.
4) I then set Microsoft Print to PDF as the defualt prionter and it works. However this is not a good long term option.
Any ideas?