Sue,
Hollow heart is influenced almost always by the prevailing growing conditions when the small tubers are forming, rather than to any specific nutrient deficiency, like Calcium (lime) mentioned previously. However calcium, magnesium and potassium are all implicated in this condition. Potassium would be the most important deficient nutrient.

Cool wet soils when the tubers are forming combined subsequently with rapid growth is a high risk situation. Almost always found in large rapidly growing tubers.

I wouldn't be too concerned about applying lime unless your soil was severely deficient.

Hope this helps