Disposal of BPR assets - three year grace period

I am aware that a time limit of three years applies to the reinvestment of proceeds into a qualifying BPR asset with BPR continuity, following the sale of an asset that qualified for BPR in the hands of the vendor.

My presumption to date has been that if the proceeds are held in cash form, an IhT event such as the death of the vendor will trigger a valid charge to IhT as far as the proceeds are concerned.

In other words, although the BPR status could be reestablished by reinvestment into a qualifying asset, should the vendor die before that has been accomplished, the cash proceeds would not qualify for BPR. That seems entirely logical to me, but an investment manager contact maintains that the cash continues to qualify for BPR.

An answer citing the relevant piece of legislation to settle this issue would be much appreciated.