@ Woofgang - claims to which they ought to give effect basicially refers to people to whom they ought to give consideration in making bequests. For example if someone makes a Will leaving out their son on the basis they have simply forgotten they exist that might be open to challenge. That is not to say they MUST include them, but they must give consideration to including them. So if they decide NOT to include them on the basis that they have already been well provided for or on the basis that they have had no relationship for the last 10 years, that will be fine.
If they are unable to ascertain the extent of their assets or unable to appreciate the claims to which they ought to give effect, but expresses a wish to leave a Will, the legal adviser would be well advised to seek medical advice as to testamentary capacity and if this is in the negative should consider an application for a statutory will.
@bednobs - yes technically your answer is correct, but the test is loss of testamentary capacity and not mental capacity (although the two are often synonymous). It doesn't follow however, that someone with dementia cannot make a Will.