Just got back to looking into this issue.
More detail:
The report in question is accessed via Site Actions|Site Settings|
under Site Collection Administration|Audit log reports|Security And Site Settings Reports|Security settings with a URL that looks like this:
SubmitFormToRunReport(' ','Security_settings.xml','');
When run the Excel spreadsheet has a column called 'Event Data' withni the worksheet 'Report Data 1' . This column will contain somethnig like this:
What is required is to turn this garbage into something useful, i.e. return the name of the group and the user who has been added/deleted to/from the group.
If I find an answer I'll post it here.