Many thanks for this.

Actually, having looked at this again, the weblink option is also blanked out when I right click on the attachment box. So even this option isn't available!

The box is blanked out. I have the latest version of thunderbird. See picture. Thanks.

Hiya - thanks for your input. The calendar is a google calendar linked into Thunderbird through two add-ons: 1. Lightning 2. Provider for Google calendar

I disabled, restarted and then enabled these add-ons, but the problem is still persisting.

So the attachment is only available if the calendar doesn't synchronise to an online calander?

Ok - thanks for this. ben