Not sure if this went through the first time, so apologies if it is duplicated.
Collette, I note that the heading to your post refers to the residence nil rate band, but in the body of your original query you refer just to the nil rate band and transferable nil rate band.
Assuming that you are actually talking about the residence nil rate band then I believe the advice from HMRC may be correct. The husband did not use his residence nil rate band, because the whole of his estate passed either to the NRB trust, or presumably to his wife. His residence nil rate band is therefore still available and can be transferred to the wife. However, the total amount of the RNRB and TRNRB available to her is limited to the value of her share of the residence, i.e. £300,000.
Paul’s comments would of course be correct if you were talking about the basic nil rate band.
I recall there is a quirk in the downsizing provisions which I believe has the effect of restricting the value of the TRNRB in some circumstances, but I can’t recall what those circumstances are and am not able to check at the moment, so may be worth researching.