PA1P or IHT402 for TNRB?

I know this topic has been discussed elsewhere on this forum but, as far as I can see, without a definite answer: if there is a thread which gives a generally accepted answer, I apologise in advance, and would be grateful if somebody would be kind enough to point me to it.

A leaves an estate valued at approx £550k
A was married to B, who pre-deceased A
B has an unused nil rate band of approx £300

so that A’s NRB & B’s TNRB = £625

In this situation, to ensure that B’s unused nil rate band is transferred to A, is it:

  • enough to complete section 7 of PA1P ?

or

does an IHT 402 also have to completed?

A reading of IHTM 43006 is that ‘a valid claim’ has to be made to transfer the unused NRB. At the same time, however, there are a lot of suggestions out there that, to avoid IHT 402s being completed for excepted estates, PA1P plays the role of capturing the required IHT information.

Any clarification would be very helpful.