This is discussed in the SDK Help files - under 'external reporting'. You need to create a class which implements IReportingEngineHost and then pass the Report instance to it, rather than calling report.Run();.

Here's an example of such an implementation:

public class SendToExcelExample : Sage.Reporting.Engine.Integration.IReportingEngineHost
    {
        private Sage.Reporting.Engine.Integration.IReportingEngineSite _site;

        public SendToExcelExample()
        {
        }

        #region IReportingEngineHost members
        public IReportingEngineSite Site
        {
            get => _site;
            set => _site = value;
        }

        public object GetService(Guid identifier)
        {
            return null;
        }

        #endregion

        public bool SendReportToExcel(Sage.Accounting.Reporting.Report report, string outputFileName)
        {
            bool success = false;
            try
            {
                if (report.BuildFileName())
                {
                    string actualFilename = report.ActualFileName;
                    var criteria = report.Criteria;
                    var exportType = Sage.Reporting.Engine.Integration.ExportType.SendToExcel;

                    Sage.Reporting.Engine.Integration.ReportingEngine reportingEngine =
                        new Sage.Reporting.Engine.Integration.ReportingEngine();
                    _site = reportingEngine.CreateInstance(this);

                    //create the export service
                    Sage.Reporting.Engine.Integration.IExportService exporter =
                        (Sage.Reporting.Engine.Integration.IExportService)Site.GetService
                        (Sage.Reporting.Engine.Integration.ServiceIdentifiers.Export);

                    exporter.Load(actualFilename);
                    foreach (Sage.Accounting.Reporting.Criterion criterion in criteria)
                    {
                        if (criterion.Values != null)
                        {
                            exporter.Criteria.Add(criterion.Name, criterion.Values);
                        }
                        else
                        {
                            if (criterion.High != null)
                            {
                                exporter.Criteria.Add(criterion.Name, criterion.High);
                            }
                            if (criterion.Low != null)
                            {
                                exporter.Criteria.Add(criterion.Name, criterion.Low);
                            }
                        }
                    }

                    success = exporter.Run(exportType, 
                        outputFileName, ExportFlags.SuppressUserInteraction);

                }
            }
            catch
            {
                success = false;
            }
            return success;
        }
    }

And here it is in use:

Sage.Accounting.Reporting.Report report =
                Sage.Accounting.Application.ReportingService.CreateReport();

            Sage.Accounting.Reporting.Criteria criteria = report.Criteria;
            Sage.Accounting.Reporting.Criterion criterion;
            criterion = criteria.Add();
            criterion.Name = Sage.Accounting.Common.ReportingConstants.CRITERIA_SALES_BALANCE;
            criterion.Low = 0.00;
            criterion.High = 20000.00;

            criterion = criteria.Add();
            criterion.Name = Sage.Accounting.Common.ReportingConstants.CRITERIA_SAL_TRAN_TYPE;
            criterion.Values = new object[] { EnumSalesTransactionType.Receipt, EnumSalesTransactionType.Payment,
                                                    EnumSalesTransactionType.Invoice, EnumSalesTransactionType.CreditNote };

              criterion = criteria.Add();
            criterion.Name = Sage.Accounting.Common.ReportingConstants.CRITERIA_SALES_REF;
            criterion = criteria.Add();
            criterion.Name = Sage.Accounting.Common.ReportingConstants.CRITERIA_SALES_SHORT;

            report.OutputMode = Sage.Accounting.Reporting.OutputModeEnum.Preview;
            report.FileName = @"Sales\Sales Ledger Transactions Report.report";
            report.Criteria = criteria;

            //pass the report to the exporter class
            SendToExcelExample example = new SendToExcelExample();
            example.SendReportToExcel(report, @"D:\MyOutputFile.xlsx");

okay noted. thank you Chris. I thought the example on the site is for pdf only