When writing thank you letters to interviewers and admissions staff following the interview is there anything specific format to which I should adhere?
Surely make it a casual letter. Show them that you had a great time and feel closer to them and to the school, so you shouldnt have to write a very formal-style letter.
Thank you letters can be casual, either handwritten or typed if your handwriting is messy, on thank you cards or regular paper. The things that are noticed are references to the connections you made with the interviewer -- to jog their memories -- and reiteration of the main reasons (1) why you want to go to that school and (2) why you would fit in there.