Exporting diary-sexps into icalendar file

[Stephen Eglen]

hi,

I have a simple file, notes.org, with the following content.  (This is
the example from the org mode manual).

----------------------------------------------------------------------
* 22:00-23:00 The nerd meeting on every 2nd Thursday of the month
  <%%(diary-float t 4 2)>
----------------------------------------------------------------------

and this entry then appears once/month in the org agenda, as expected.
However, when I export to ics, the notes.ics file is effectively empty:

----------------------------------------------------------------------
BEGIN:VCALENDAR
VERSION:2.0
X-WR-CALNAME:notes
PRODID:-//Stephen Eglen//Emacs with Org mode//EN
X-WR-TIMEZONE:BST
X-WR-CALDESC:
CALSCALE:GREGORIAN
END:VCALENDAR

----------------------------------------------------------------------

I have org-icalendar-include-sexps set to t

Any clues what could be going wrong?

Stephen