An owl bearing a letter stamped with the St Mungo's seal arrives the next day. The contents reveal a piece of parchment with the St Mungo's logo as the letterhead - certainly more official and formal than the recipient has probably seen.Johann Storm
2B, House of Atreus
Knightsbridge
London
Potions and Plant Poisoning
St Mungo's Hospital for Magical Maladies and Injuries
London
Dear Mr Storm,
15 November 2010
You have been booked in for the 1.30pm slot on this coming Thursday, 18 November 2010, with Healer Arcturus Hollingbury for consultation. Please keep this letter with you as a reminder of your appointment as well as proof to show the clerk at the front desk. Should you be late or unable to come, please let the hospital know in advance so that we can arrange another time between you and your assigned Healer.
Thank you.

PS: Please ignore the formal tone, it's required. Not in postscripts, however!
PPS: Well done.