Late Claim for Transferable Nil Rate Band

Does anyone have experience of bringing a claim for transferable nil rate band beyond the 24 month deadline from date of death? I understand HMRC have discretion to accept a claim by the PRs out of time but have experienced another estate where they declined to do so (despite in my view quite compelling reasons for the delay!).
We have just been instructed by the PR of an estate where the deceased died 26 months ago. TNRB from his predeceased wife’s estate is needed to avoid the estate being taxable. Under S8B IHTA 1984 “the permitted period” means–
(a) the period of two years from the end of the month in which the survivor dies or (if it ends later) the period of three months beginning with the date on which the personal representatives first act as such

Does anyone have experience of what HMRC deem to be ‘first acting as such’? Our terms of business to administer the estate have only just been signed, but the PR has been insuring the deceased’s house and paying utility bills for two years now.

IHTM43007 gives HMRC’s view on when an exor has first acted as follows

Where an executor undertakes acts which show an intention to take on the office of executor and which would prevent them from renouncing probate, that is the point when they have ‘first acted’ as an executor.

I believe the question of late claim has been raised on the forum before. My only experience involved a case where the executor’s parents and wife all died within a few months of each other and as a consequence the executor became depressed and unable to deal with anything. We did mange to get the late claim through but only after involving the GP and even then HMRC pushed back initially.