I know there is a lot of confusion surrounding this.
Jeremy Vine on BBC R2 (can relisten via BBC Sounds) at start of his programme is discussing differential symptoms with the developer of Covid symptom study app Tim Spectre, Professor of Genetic Epidemiology, KCL.
In summary, he states if you have any of the 3 classic symptoms - get tested. He refers to a persistent cough and a tested (rather than to the touch) fever.
Mild symptoms including runny nose or swollen glands, unlikely to be Covid.
He stated in 80% of positive cases, severe fatigue and severe persistent headaches were also present.
Shortness of breath is an indicator in adults but usually after a few days.
Diarrhoea is positively associated with Covid, especially in children, but this is usually accompanied with one or more of the 3 main symptoms and/or the severe fatigue and persistent headaches.
I appreciate others on here may have experience of positive cases with milder or different symptoms but personally I found his information clear and would help me decide if a test for me or DC was necessary.