Sir Alec -tiny strokes can lead to vascular dementia - my mother has both Alzheimers and vascular as a result of one she had three years ago. The CT and MRI showed both of them up.
Forgetting is one thing, the second is in action - for example, as it takes grip a common "feature" is that they often flay out accusing family or friends of taking things that they can not find, like money and jewellery.
Missprim, if you know her family, I would strongly encourage them to get any powers of attorney and wills etc sorted out know while she/he still has reasonable faculties.....it is a legal nightmare when it is too late.