It's worth noting that the Ilford sequence for optimum permanence calls for a 20-minute final wash in running water. The sequence with the five-minute final wash is for a much lower standard of permanence (or a typographical error!). I certainly wouldn't trust a five-minute wash to do the job!
In any case, testing is your friend here; HT-2 for residual hypo...
Best,
Doremus
I'm affraid this is not correct. The following text precedes the Ilford archival sequence (5-minute final wash) in all their documents: When optimum permanence is needed, perhaps for archival storage of prints, the following fixing and washing sequences at18–24°C/65–75°F (including wash water) are recommended using ILFORD WASHAID.
It's amazing how low the thiosulfate levels go after HCA and a 5 min wash only as can be seen from these two documents:
http://www.film-and-darkroom-user.org.uk/forum/showthread.php?t=296 (Part one)
http://www.film-and-darkroom-user.org.uk/forum/showthread.php?t=344 (Part two)