SQL Query Select, IF, Case doubt

Dear Kind Friends,

i have working query , 

SELECT  [R2IGTNo], [ChtrONDteTm],[ChtrOffDteTm],CAST(DATEDIFF(second, ChtrONDteTm, ' '+ChtrOffDteTm) / 60 / 60 / 24 / 7 AS NVARCHAR(50)) + ' W '    + CAST(DATEDIFF(second, ChtrONDteTm, ChtrOffDteTm) / 60 / 60 / 24 % 7 AS NVARCHAR(50)) + ' D '    + CAST(DATEDIFF(second, ChtrONDteTm, ChtrOffDteTm) / 60 / 60 % 24  AS NVARCHAR(50)) + ' H '    + CAST(DATEDIFF(second, ChtrONDteTm, ChtrOffDteTm) / 60 % 60 AS NVARCHAR(50)) + ' m & '    + CAST(DATEDIFF(second, ChtrONDteTm, ChtrOffDteTm) % 60 AS NVARCHAR(50)) + ' s' as ChtrONDuratn, [ChtrOFFRemark], [ChtrOFFby] FROM [tbl1RMWeighingDetails]

Result come like this (some columns omitted like R2IGTNo,..)
0 W 0 D 18 H 48 m & 0 s

What i need is

18 H 48 m

That is if some value is there only it Should Show