Very useful, Alex29, thank you.
Mind you, I think a letter only expresses your desires - it may clarify things, but the executors can override it if they want to.
Whereas a codicil is a proper extra will, tagged onto the existing one. You certainly don't have to make a new, full-blown will when things change: for example, you'd probably make a brand new will when you had children, but you might only need to make a codicil if one of them turned out to have special needs and you wanted to make doubly sure they were okay if you went under a bus.
Note - I am NOT a lawyer! Do take proper advice.
And on intellectual property, the Society of Authors have a leaflet:http://www.societyofauthors.org/guides-and-articles
Don't forget that copyrights, like other intellectual property, have a value for the purposes of probate, even if it hasn't been realised yet. Yet another reason to think about who might handle your literary estate - chaffering with HMRC over the value of a book which has earned you an advance and will earn in the future but isn't exactly like a painting or a house or a bunch of shares, can get complicated...