Best case, the gift is voidable rather than void so the property is currently owned by mother’s estate and has to be included on the IHT return. If a taxable estate, you could include a claim as a potential liability and attempt to value the claim based on its merits (with detail in the additional information to cover yourself).
I don’t know whether this could have a knock on effect when answering standard questions on the sale.
More widely, I haven’t seen the case to which you refer, but any claim here would like turn on whether the lack of knowledge amounted to a “mistake”. To my mind, the law is still pretty ambiguous in this area (ignorance of the illness doesn’t count but an incorrect assumption that mother was in good health might) so a case could come down to how the Judge felt about the fairness of the circumstances. My gut instinct is that such a claim would be pretty ambitious so I would not value it very highly but others may disagree.
Osborne Clarke LLP