In the UK it's unlawful to ask candidates about their health or medical history. If they have a gap due to medical reasons then questioning the gap means you put them in a position where they will either have to disclose information they by law are protected from disclosing, or flat out lying, which is not a good base for mutual trust. To protect people from this, this question should just be removed from the process altogether. Just check their references and DBS records instead.