There are certain formalities which have to be complied with for a Will to be valid. These are set out in section 9 of the Wills Act 1837 and describe the physical process which must be gone through when a Will is executed for it to have what is generally referred to as ‘formal validity.’
Certainly there are other considerations which must be taken into account, such as the Testator’s capacity, but if the provisions of section 9 are not complied with, the Will will not be valid in any case.