[Proj] Proj4 transform from WGS84 to UK Northings and Eastings help needed ...

[Penny Bamborough]

Hiya all

I have some Navteq data, which I believe is in WGS84 decimal degrees. 
I am using an ESRI product (which uses proj4) to project the data 
into british national grid northings and eastings. However, when I 
project the data, the coordinates appear to be between 100 to 200m 
out from where I would expect. I use the following settings for the 
proj4 translation:

proj=tmerc
a=6378137.0
b=6356752.3141
k_0=0.9996012717
ellps=airy
lat_0=49d0n
lon_0=2d0w
x_0=400000
y_0=-100000

Would anyone have any suggestions as to why we receive this position 
offset error ?

Penny