SQL Statement

I would question the use of an approximate data type (FLOAT) for the transaction quantities instead of an exact type such as NUMERIC(15,8).  If you're not dealing with fractions of items you might even get away with INT.

I would also ask why PartDesc is in this table.  Do the descriptions vary over time?  If you use "GROUP BY PartNo, PartDesc" you'll get multiple rows for the same part number, which I assume is not desirable.  Database normalization rules would suggest a separate table with (PartNo, PartDesc).

But back to your query, I'm assuming the Opening Stock is StkInHand from the earliest transaction for a PartNo, and PartDesc is taken from the most recent.  The first version does it with one subquery.

SELECT  PartNo, 
        PartDesc = MAX(CASE WHEN Latest = 1 THEN PartDesc END),
        OpStk = MAX(CASE WHEN Earliest = 1 THEN StkInHand END),
        NewPur = SUM(NPurQ), 
        QtyIss = SUM(IssQty),
        BalStk = MAX(CASE WHEN Earliest = 1 THEN StkInHand END) + SUM(NPurQ) +  SUM(IssQty)
FROM (
    SELECT  PartNo, PartDesc, StkInHand, NPurQ, IssQty, 
            Earliest = ROW_NUMBER() OVER (PARTITION BY PartNo ORDER BY RepDate), 
            Latest = ROW_NUMBER() OVER (PARTITION BY PartNo ORDER BY RepDate DESC)
    FROM dbo.StoTrans
) st
GROUP BY PartNo

The second version uses separate subqueries and is IMHO more readable and maintainable.  It may perform differently than the first if the table gets very large, but you'd have to have a large table for testing to prove it.

SELECT  trans.PartNo, pt.PartDesc, OpStk = InitialStock, NewPur, QtyIss, BalStk = InitialStock + NewPur + QtyIss
FROM (
    -- Get aggregate values from all transactions
    SELECT  PartNo, NewPur = SUM(NPurQ), QtyIss = SUM(IssQty)
    FROM dbo.StoTrans
    GROUP BY PartNo
) trans
INNER JOIN (
    -- Find earliest StkInHand
    SELECT  PartNo, InitialStock = StkInHand,
            Sequence = ROW_NUMBER() OVER (PARTITION BY PartNo ORDER BY RepDate)
    FROM dbo.StoTrans
) init ON trans.PartNo = init.PartNo AND init.Sequence = 1
INNER JOIN (
    -- Find most recent PartDesc
    SELECT  PartNo, PartDesc,
            Sequence = ROW_NUMBER() OVER (PARTITION BY PartNo ORDER BY RepDate DESC)
    FROM dbo.StoTrans
) pt ON trans.PartNo = pt.PartNo AND pt.Sequence = 1