Recent tests showed my 9am plasma cortisol level was high (644 nmo/l ref range 170 - 550nml/l) and 11pm salivary cortisol normal. what can this mean?

Stress: Chronic stress can cause elevated cortisol--although it could also be due to your diabetes. I suggest DHEA (dehydroepiandrosterone), 25 mg in the morning. DHEA should help to normalize your cortisol, and help you handle stress. I also suggest 1,000 mg mastic gum (available on line or from a health food store) for your abdominal distress--may be able to discontinue Rabeprazole.