@jared-wiltshire thanks Jared.
No problem, I have committed a fix which will be included in the next Mango core release. The bug was in the core, not the module.
As an alternative work-around you could override or create a new email template .ftl file and replace the <@img logo="true"/> with <@img src="logo.png"/>
To create a new email template for your Excel report -Copy the supplied template to your overrides, e.g. cp /opt/mango/web/modules/excelReports/resources/ftl/excelReports/reportEmailBody.ftl /opt/mango-data/overrides/ftl/excelReports/myTemplate.ftl Restart Mango Edit your report to use the new template file