Size of SQL Server 2008 R2 database question

Hi

I am currently in the process of taking a cut-down copy of our production database and installing it onto SQL Server Express.  I have taken many steps to shrink the database (removed tables, truncated existing data etc) as much as I can but can only get it to 15GB.  The target is under 10GB.  I have used the following query to see where the data is used most:

SELECT
    t.NAME AS TableName,
    i.name as indexName,
    sum(p.rows) as RowCounts,
    sum(a.total_pages) as TotalPages,
    sum(a.used_pages) as UsedPages,
    sum(a.data_pages) as DataPages,
    (sum(a.total_pages) * 8) / 1024 as TotalSpaceMB,
    (sum(a.used_pages) * 8) / 1024 as UsedSpaceMB,
    (sum(a.data_pages) * 8) / 1024 as DataSpaceMB
FROM
    sys.tables t
INNER JOIN
    sys.indexes i ON t.OBJECT_ID = i.object_id
INNER JOIN
    sys.partitions p ON i.object_id = p.OBJECT_ID AND i.index_id = p.index_id
INNER JOIN
    sys.allocation_units a ON p.partition_id = a.container_id
WHERE
    t.NAME NOT LIKE 'dt%' AND
    i.OBJECT_ID > 255 AND
    i.index_id <= 1
GROUP BY
    t.NAME, i.object_id, i.index_id, i.name
ORDER BY
    (sum(a.total_pages) * 8) / 1024 desc

Having added all of the rows up in the total space used column though, it only amounts to 7GB.

Since my cut-down database copy is 15GB and the table total storage is 7GB, this leaves 8GB unaccounted for.  My question is how can I find out where the other 8GB is being used please?

Thanks.