One of the writing topics most usual at UNED exams is" writing a complaint letter". Imagine you have booked a flight and once you are in the airport the staff tells you that you that there is overbooking and you cannot fly till next day. Explain that you needed to arrive very early in the morning the first flight you had was for next day the 13.00.
Don't worry to use all the linkers and expressions suggested in the book (page 122) .
Here you are some useful websites:
Remember to write your tasks as a "comment".