Symptoms to watch out for

Bowel habit:

A recent and persistent change of bowel habit (alternating diarrhoea and constipation, for example);

Rectal bleeding:

Even if you think it's piles, have your doctor examine you;

Abdominal pain:

Especially in an older person, persistent abdominal pain requires urgent investigation;

Unexplained anaemia:

A low and otherwise unexplained blood count.