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The time limits vary but for fraud there is no time limit. You would need to establish undue influence.
The video evidence shows that he was very ill but it would also need to convince a court that your father's condition was such that undue influence would have succeeded. The solicitor who attended will testify that your father had capacity.
As such it is still possible to challenge the Will at this stage but you have a tricky piece of litigation on your hands.
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