Convert a collection of Byte arrays to an excel workbook containing multiple worksheets?

Hi All - I have a collection of byte arrays - each byte array has been returned from the MS reporting services web service Render() method (rendered in Excel format obviously)

What I'm trying to do is to save each of these byte arrays as an individual excel worksheet, and then combine these excel worksheets into a single excel workbook. This (ideally) needs to be done in memory - without saving anything to the hard drive. Once the actual workbook containing all the worksheets has been created, the workbook (only the workbook) will then be saved to the hard drive.

Can anyone point me in the right direction or offer a code sample? I can't seem to find any info at all on how to do this (in memory at least)!

Thanks