I didn't write a letter at all, just told people face-to-face... I had to get a letter from the midwife with my due date though, and saying how long I was fit to work up to (my work has a policy saying you have to take leave from maternity 34 weeks unless you are medically OK'd to stay longer).

I would just write what your due date is, and how long you would like to take leave for (and from what date).

Hope it all goes well for you!