In theory, the repeat box should only be grayed out if you're editing a single instance (=not all events belonging to the repetition). After double-clicking on an unchanged instance (!) of a repeating event, you'll be asked whether you wish to edit only the event you clicked at, or all events. Select all events in order to change repetition settings.

If you already changed a single instance of the event, Thunderbird always assumes that you want to edit this particular instance, and thus does not permit to edit repetition settings.

While a logical explanation for what's happening (or rather the reasoning behind it) , this doesn't answer my question.

While the event in question may have started off as one of a repeating series, it has got seperated/orphaned - possibly after being copied - from the repeating series: as the 'click here for details' is also non-operational it's difficult to tell.

If the event is a modified single instance of a repeating series, is there a means of converting it to a non-repeating, stand alone event (which is what it effectively is)?

Hm. I played around a bit and I think I was able to reproduce your issue. It seems as if it's a bug in Thunderbird: when cut-pasting a recurrence instance, Thunderbird inserts the unchanged repetition exception as a new event. Depending on the calendar provider, this might lead to an error while pasting, or (as with local calendars) to the creation of an invalid event. Technically, this event contains a recurrence-id (and thus is marked as repetition instance), but no parent or recurrence rule. This leads to the recurrence controls being both disabled and not displaying a description of the rule.

Sadly, I don't know of a way to fix this issue directly. As a workaround, you can copy the event (select and right-click -> copy), and then press paste in a text editor of your choice. You should see a technical representation of the event (starting with BEGIN:VCALENDAR). Search for a line starting with RECURRENCE-ID, and delete that line. Then, copy all the text into the clipboard and paste it again in Thunderbird. You should then have a new instance of the event, but without the issue – the original, problematic event can then be removed.

Note: I tested this on linux, I'm not entirely sure the clipboard works the same way on other platforms. But give it a try. Alternatively, you could export and re-import, but then you need to mess with all other events of the same calendar, which is more complex than deleting a single line.

Update: this is a known bug (since 2009!), see https://bugzilla.mozilla.org/show_bug.cgi?id=536796

Yes, there is a (long time) known bug 536796 (https://bugzilla.mozilla.org/show_bug.cgi?id=536796) that is probably the cause of the problem: I was wanting a means of dealing with the result.

But a bit of playing about did lead me to a slightly simpler workaround: using 'convert to task', and then 'convert to event' seems to do the trick - 'convert to' is rather 'copy as', so you end up with the original event and a task to delete.