Fixing up a corrupted toast table

I have an older 7.4 installation which is giving this error
during pg_dump:

pg_dump: ERROR: missing chunk number 0 for toast value 25923965
pg_dump: SQL command to dump the contents of table "element"
failed: PQendcopy() failed.
pg_dump: Error message from server: ERROR: missing chunk number 0 for
toast value 25923965

(slightly edited, but not in any significant way).

I believe that it's a known issue that occasionally causes this (this
is not a bug report, and I'm quite aware of point upgrades and so on)
and the server will likely be updated to 8.1.current within the month.

I'd like to make the current problem go away, though, perhaps by
deleting the relevant row in the element table. I'm not quite sure how
to go about that, though. Could anyone point me in the right direction?

Cheers,
Steve