the first hall mark is the Town mark, corresponding to the mark of the Assay Office that has verified the piece (on yours it is Sheffield)
the second hall mark is the date mark or the makers mark (sorry but i can not see it clearly but i think its a makers mark)
the Lion passant guardant certifiying the silver quality (yours is for sterling) if the face is looking at you it was before 1820 if it is sideways it is after 1820