The ticket type called Open Return is now called Anytime Return, though the code '
SOR' (or 'FOR' for 1st class) is retained. If this is the ticket type you have, then yes you can make the outward journey within 5 days and return within one month. This validity is usually printed on the ticket.
However some booking engines refer to the term "open return" to mean any ticket type in the category Off Peak or Anytime. If your ticket is an Off Peak Return (
SVR) ticket, then unfortunately you do have to
commence the
outward journey on the date specified on the ticket. The return journey can be made any day within one month.
Some people refer to the term "open return" interchangbly with the term "period return" to mean any ticket type in the category Off Peak or Anytime that
doesn't have the Day suffix. Your ticket won't have the Day suffix, however the lack of the Day suffix doesn't change the fact that the outward journey has to be made on Day 1.
Your only option, if you have an Off Peak Return, is to obtain a refund, less £10 admin fee, and purchase a replacement ticket. And yes as the ticket office is now closed, you would have to wait until it re-opens.