it depends on your layout and what you are talking about in your next paragraph.
but i used "I am writing to you regarding a temporary placement for work experience within the ---- hospital in the ---- department." then i talked about my academics, then why i needed the placement and what would i learn from it and then i said that i understood the busy schedule of the hospital and didnt mind when the w.e. was. finally i ended it with "I hope you will carefully consider allocating a work placement for me at the hospital and I look forward to hearing from you soon."