Just a little thing....
At the time of this post EGTG was giving
METAR - 2008/01/13 11:20 EGTG 131120Z 19021G31KT 9999 BKN022 11/08 Q1000
Your website has classified this as marginal VFR which has a vis category of 5-8km, yet the metar is giving all the 9's. Granted that the metar cloud base is in line with the ceiling that Marginal VFR category has assigned to it.
So is your algorithm using either of the values to categorise?