SQL, Incorrect syntax on CASE statement near the keyword 'FROM'

Question

I am trying to get the below code to work, individually the two pieces of code (in the WHEN part and the ELSE part) work but when used in this CASE statement I get an error

"Incorrect syntax near 'CAST', expected 'AS'." error.

Basically if the WHEN statements code is equals to or greater than 24 then use the THEN statement if its is under 24 then use the ELSE statement.

I cannot seem to get this to work after trying for several hours any indication as to where I am going wrong would be greatly appreciated.

SELECT CASE 
        WHEN 
            (convert(float,datediff(mi, start_work, end_work))/60) >= '24'
        THEN
            (convert(float,datediff(mi, start_work, end_work))/60)
        ELSE
            (CAST(convert(varchar(2), dateadd(minute, datediff(minute, start_time, end_time), 0), 114) 
            * 60 + RIGHT (convert(varchar(5), dateadd(minute, datediff(minute, start_time, end_time), 0), 114),
            CASE WHEN CHARINDEX(':',convert(varchar(5), dateadd(minute, datediff(minute, start_time, end_time), 0), 114)) > 0
            THEN LEN(convert(varchar(5), dateadd(minute, datediff(minute, start_time, end_time), 0), 114))-3
            ELSE LEN(convert(varchar(5), dateadd(minute, datediff(minute, start_time, end_time), 0), 114))
            END) AS decimal) / 60
FROM  NDB.dbo.statusa 
INNER JOIN NDB.dbo.details ON statusa.vkey = details.vkey
INNER JOIN NDB.dbo.chegu ON statusa.ckey = NDB.dbo.chegu.gkey
WHERE start_time!= end_time AND string1 = Visit_Id and NDB.dbo.chegu.name = 'loft'
     AS [Working]

Answer 1

you need to close your case statement

 case when ... then ... else ... end

Answer 2

There should be an END before the FROM clause and you should also remove( before CAST.