Firstly are you sure that you meet the established presence criteria?
which is that:
you have completed a course that was over 6 months long in the last 4 months
or
you are continuing on a course, and have completed at least 6 months of that course and have been studying in the last 4 months
AND
you are currently here on a student or tier 4 visa
(i.e. if you are here on a PSW visa for example you won't be treated as an established student).
If you meet this criteria then you have to show 2 months maintenance for 28 days - so £1600 (2 x £800) if you are studying in inner London.
See policy guidance:
http://www.ukba.homeoffice.gov.uk/sitec ... dance1.pdf