The Wills Act 2007 sets out what you need to do to make a valid will. The requirements are not complicated, but they are strict. Amongst other things, your will must be dated, and be signed by the will-maker in the presence of two witnesses, who must also sign the will. Each party must initial each page. A person who may benefit under the will (eg. a beneficiary or professional executor) should not be a witness, otherwise they will lose their right to a benefit under the will.
What many people do not realise is that a perfectly valid will is rendered invalid if you get married or enter into a civil union, unless the will specifically states that it is made in contemplation of that marriage or civil union (Section 16, Wills Act 2007). Similarly, if a will is made during a marriage or civil union and then the relationship is legally dissolved, some parts of your will may then be invalid (Section 19, Wills Act 2007).