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The normal way of revoking a will is either expressly by a subsequent will or by physical destruction either by the testator or by someone on his express instructions.
Unfortunately, T's letter purporting to leave his Estate to Charity is not a valid Will, in that only one party has witnessed it. If it had been executed correctly, then this would have automatically revoked his "previous" Will.
As it stands, therefore, the question needs to be- was the Will revoked or not?
I hope you appreciate I can't give a "Yes" or "No" answer to this, as it is something that would need to be decided in Court. To show that the Will was revoked you would need to provide as much evidence as possible to not only show that T intended to revoke his Will but also produce some evidence of the T's state of mind at the time - if known (to confirm he was of sound mind). You will therefore require copies of all the correspondence between T and his Solicitor and T and his Executor. I cannot find any specific authority on similar facts and it will be a matter of convincing a judge the Will was revoked.
On a side point, please note that if the Will is deemed to have been revoked, then T's Estate would pass in accordance with the terms of Intestacy (ie to his next of kin). As mentioned above the Charity will not receive a penny, whichever outcome it is.
I hope this helps you and set sout the legal position.