The International Student Services at your UK University should help you with all your queries.
Your agent is actually right about the immigration health surcharge. You have to pay 225 which includes the additional 4 months of visa that you get - again get a confirmation from your University.
"For example, if you are planning on studying a 12 month Masters degree at Exeter then the actual length of your Tier 4 visa is 17 months. Therefore you would be charged £150 for the first 12 months and £75 for the remaining 5 months thus £225 in total."
Above excerpt taken from University of Exeter website -
http://www.exeter.ac.uk/internationalst ... surcharge/
Works out as 12 months course + 1 month visa (before course start date) + 4 months additional visa (after course end date) - CONFIRM WITH YOUR UNIVERSITY.
Regarding the application form: Make sure you correct the form and sign against the correction (best to correct it than leaving it with an incorrect answer).
Hope that helps.