If the deceased had a spouse (or civil partner) who died before him/her, and leaving everything to them, the IHT allowance will be TWO lots of £325k (= £650k), meaning that there's no IHT to be paid at all.
Otherwise the allowance is just ONE lot of £325k, meaning that IHT will need to be paid on the excess £300k. That amount, at the current rate of 40%, is £120k.