For a 14-month-old with 3 days of fever plus vomiting and diarrhoea now, a normal GP clinic is a reasonable step, but if he seems very lethargic, is not passing urine, or the vomiting gets worse, I would go straight to A&E instead of waiting. The combination of symptoms in a baby this young can lead to dehydration quite fast, so it is worth being cautious and having a doctor assess him properly rather than monitoring at home.