There might be a problem CHOCALCHEMY has a ok result created by the CO with the GSAK macro but fails with only 213 of 221 squares.
the problem might be that straight lines between point in WGS84 is not the same as lines in BNG.
There was a discussion of a similar problem about another checker.
If the a straight NS EW line has the same coordinares unchanges in booth systems the checker sould work. If they are not then the checker has to be done in BNG