Here are some useful phrases from my most recent 'General Training Task 1' sample answer (the complaint letter):
- bring something to someone's attention
- a significant error
- the mistake relates to
- it is stated that... when in fact it is...
- something is due to happen
- sorely disappointed (very disappointed)
- an outing (a trip or day out)
- this could lead to complaints
- bad publicity
- rectify this problem
- it is vital that
- give accurate information
- I trust you will act quickly