I handed notice in my notice face to face to my line manager. I sent an email so it was in writing and included my last working day. She responded email saying thanks, lovely working with you, the usual. That was six weeks ago. And when I mentioned my leaving date to a colleague, line manager admitted she hadn't actually read my email and they want me to stay past this date as it's taken them 4 weeks just to get my job advertised :/ I want to leave on the date on my email. By replying to my email my line manager accepted that as my last working day. What can I do?