Just another suggestion.
If you are feeling fatigued or likely to be fatigued on a flight, email or fax your crewing department with a copy to the Chief Pilot informing them that in your view you are fatigued or likely to be so.
You are quite prepared to operate the flight if ordered to do so but will not report for the flight unless instructed but it would be contrary to section ???? of the company ops manual.
Follow this up after the next day with a notification of sickness with the reason for sickness as fatigued.
Keep a copy of the email or fax and if you get any comeback forward it to the CAA FOI.