Apparently Platform 9 and three quarters is between Platform 4 and 5 at Kings Cross in a quirk of location.
JK Rowling admits that she actually made a mistake when she placed Platform 9¾ at King's Cross Station: "I wrote Platform 9¾ when I was living in Manchester, and I was actually thinking of Euston. So anyone who's been to the real Platforms 9 and 10 at King's Cross will realise they don't bear a great resemblance to the platforms nine and ten as described in the book, and that would be because I was thinking of Euston at the time (HPM)."
The platform actually used for filming at King's Cross Station is Platform 4. Hagrid and Harry are also filmed walking over the pedestrian bridge across the tracks.
