Letter wording:

Your statutory rights in relation to Sunday shop work:

You have become employed as a shop worker and are, or can be, required under your contract of employment to do the Sunday shop work that your contract provides for.

However, if you wish, you can give notice, as described below, to [ ] and you will have the right not to work in the shop on any Sunday on which it is open once three months have passed from the date on which you gave the notice.