Do not assume an extension. Use the actual notice and applicable process to address the situation, while keeping the employer’s timeline documented.
Coordinate the two calendars honestly
A delayed internal approval is not a government decision to give more time. Track what was requested, what can be supplied and what remains unresolved without promising that the case will finish on a fixed date. Put both calendars on one page and mark where they collide.
On one side, the date of the official notice, the period it allows and the date a response must be made. On the other, the employer's internal steps, who has to approve at each, and the realistic elapsed time for each approval. Hypothetical example: a drainage and site servicing contractor's records committee meets monthly, and the next meeting falls after the response date on the notice.
Knowing that now allows three things: a request that the matter be dealt with outside the ordinary cycle, a parallel search for alternative evidence, and a question to counsel about what the applicable process permits when material is genuinely outstanding. Knowing it a week before the deadline allows none of them. Record what was requested, what can be supplied and what remains unresolved, and do not represent to anyone that a decision will arrive on a fixed date.
Record the date each document was actually received rather than the date it bears, since a letter written in one month and released in another leaves a gap someone will ask about.