Sunday working opt out question

[welshmammy2009]

I work in retail and have worked for the same company for 10 years. When I started with the company I wrote a letter to opt out of sunday working and gave it to my current manager. 10 years and 6 different managers later and the letter seems to have disappeared.
Now my employer is requesting me to work sundays. Do I need to write another letter to opt out and give 3 months notice or is it their fault for losing the letter?