For example, the testator usually has to sign at the end of the document and their signature is then witnessed by two or more witnesses who were present when the testator signed.
The person trying to prove (or ‘propound’) the Will can prove ‘due execution’ by showing that these provisions were complied with.
The person contesting the Will may argue they were not, perhaps because the signature has been forged or the witnesses were not present when the testator signed.
Note that sometimes mistakes as to execution can be rectified – for example, in Marley v Rawlings and another [2012] EWCA Civ 61, a case involving a husband and wife that signed each other’s mirror Will, the Supreme Court ultimately held that the issue could be rectified under section 20(1)(a) of the Administration of Justice Act 1982, because in effect it was a clerical error.
Contrast this with Barrett v Bem and others [2012] EWCA Civ 52 in which the Court found that the Will, which had been signed for the testator by their sister, was invalid due to insufficient evidence that the sister was signing the at the testator’s direction. Where a Will is signed at the testator’s direction, some sort of positive communication (verbal or non-verbal) by the testator is needed and there was no evidence of this.
The two cases are a good comparison to show the difference between a clerical mistake (which was the fault of the solicitor who handed each of the parties the wrong Wills) and a Will that has not been executed in accordance with legislative requirements.