What is a letter of wishes?
A letter of wishes is an informal, private document sitting alongside a will or trust that explains the reasoning behind it. It is not legally binding, but executors and trustees are expected to read it and take it into account when exercising their discretion.
Its main use is guiding discretionary trustees, who have wide powers and little else to go on. It can also record funeral preferences, personal messages, the location of documents, and — importantly — the reasoning behind unequal treatment of children, which is valuable evidence if the will is later challenged.
Because it is not a formal document, it can be rewritten as often as circumstances change, with no witnesses and no cost. That flexibility is the point: it holds the reasoning that would be inappropriate, or unwise, to put in a public document.
It should be kept with the will but plainly marked as separate from it — and, unlike the will, it should be reviewed every few years, because a letter of wishes written for a family that no longer exists is worse than none.