If there is no other witnessed will then any existing properly made will is "the one" and cannot be revoked by a letter. All estates, no matter how small must be registered for probate and the grant made. So far as I know, there are no exceptions a lthough small estates are less complex.
So was the estate registered for probate? Was there a will produced or was the intestacy procedure followed?
Not getting a grant of probate is essentially the same as not declaring income to the taxman, regardless of whether or not tax is due and hmrc take a dimmish view. Be aware though that if there was no will ie the one you knew about had been destroyed, then your wife might get less under intestacy law......