Having worked in recruitment if you waffle they wont read it! be precise and to the point keep the information restricted to the minimum you know how some people talk without actually saying anything?....well you dont want to do that when you write.
I would take each requirement on the person specification and match your compentence to that requirement giving clear evidence of why you think you have the knowledge, skills or experience. Surefire way to get an interview (as long as you do meet the essential requirements)