Is there a legally mandated filing fee and/or deposit?

Presidential  Elections Act, No.15 of 1981

14.

(7). Every reference undersubsection (6) on any objection taken to the nomination of the candidateconcerned -

(a). shall be made by petitionin writing which shall be free of stamp duty;

Source:

http://aceproject.org/regions-en/countries-and-territories/LK/case-studies/presidential-elections-act-no-15-of-1981.pdf/view