;WITH cte AS
(
	SELECT *,
	ROW_NUMBER() OVER (PARTITION BY NHS_No ORDER BY StartDate) AS RN
	FROM YourTable
)
SELECT
	a.NHS_No,
	a.Cluster_no,
	a.StartDate,
	COALESCE(a.endDate,
		CASE 
			WHEN DATEDIFF(dd,COALESCE(b.startDate,'19000101'),a.startDate) >
				DATEDIFF(dd,a.startDate,COALESCE(c.startDate,'19000101')) THEN 
					b.startDate
			ELSE c.startDate
		END) AS EndDate
FROM
	cte a
	LEFT JOIN cte b ON b.RN+1 = a.RN
	LEFT JOIN cte c ON c.RN-1 = a.RN
ORDER BY
	a.NHS_No,
	a.ClusterNo,
	a.StartDate;