INSERT INTO C (MeterId,PresRecTimestamp,PresUnits,PresVal)
SELECT DISTINCT
A.FlowMeter,
B.RecordTime,
substring(B.FormattedValue, CHARINDEX(' ',B.FormattedValue)+1, LEN(B.FormattedValue)),
substring(B.FormattedValue, 1, CHARINDEX(' ',B.FormattedValue)-1)
FROM  B
INNER JOIN  A ON  A.Id = B.Id
WHERE
NOT EXISTS (SELECT * FROM C  WHERE
C.MeterId = A.FlowMeter            AND
C.PresRecTimestamp = B.RecordTime)