I have a VB.Net App which I have built using a CrystalReportViewer to view reports built in CR11

Users have a runtime version of CR8 on the machines and when they try to export a report from the app to Excel, using the inbuilt export button on the viewer, they get the following error message:

Error in File C:\windows\temp\{blah blah blah}.rpt:
Invalid export DLL or export format

I'm pulling my hair out here, does anyone know how to resolve this issue

Thanks