Contact the Planning Officer who will be dealing with the application and explain the situation. It is not uncommon. In my experience of this situation, the Planner indicated when he/she would be inspecting the site notice, giving the applicant a timeframe to ensure the notice is in situ e.g. the Planner said they would be out the following afternoon to check the Notice. When the Planner subsequently inspected the site, they confirmed the notice was in place and took a photograph.
Thereafter if a member of the public rang in to complain that there was no site notice in place, the Planner was able to say that "on the day of site inspection", the Notice was in place and therefore the application is still considered to be a valid one.