Iv read online that Tesco living sausages are only 1 syn each, but where is there a sausage that has 20 calories?

You are right - the Tesco Healthy Living Cumberland Sausages chilled (57g each) are only 1 syn each.

The syn values don't go purely based on calories. Syn values are based on calories, protein, fat, carbohydrates and the ingredients.
The 20 cals per 1 syn rule only applies to foods that do not contain any form of free food, therefore you can't really apply them to low fat foods containing meats, etc.