Here's what I ended up with, cheap and happy, the spreadsheet shall live on...
Microsoft.Office.Interop.Excel.Application objExcel = new Microsoft.Office.Interop.Excel.Application();
Microsoft.Office.Interop.Excel.Workbook objWB = objExcel.Workbooks.Open(strPath + strSourceFile);
objWB.RefreshAll();
string strHTMLFileName = strTempPrefix + Guid.NewGuid().ToString() + strHTMLExtension;
objWB.SaveAs(strPath + strHTMLFileName, Microsoft.Office.Interop.Excel.XlFileFormat.xlHtml);
objWB.Close();
Response.Output.Write(System.IO.File.ReadAllText(strPath + strHTMLFileName));