Fai la somma prima della formattazione
SELECT
ATM,
CONVERT(VARCHAR(10), SUM(DATEDIFF(Minute, TicketRaisedOn, ClosedOn)) / 60)
+ '.' +
RIGHT('00' + CONVERT(VARCHAR(2), SUM(DATEDIFF(Minute, TicketRaisedOn, ClosedOn)) % 60), 2)
FROM Ticket
GROUP BY ATM
Sql Fiddle:http://sqlfiddle.com/#!3/eca01/1