coordinate system - Ordnance Survey data to WGS-84

Recently I have reprojected Ordnance Survey data from British National Grid (ORD SURV GB) to WGS-84 using ArcGIS 9.2. After re-projecting I have found some shift in the data. Can any body guide me what i am doing wrong?

Thanks for you reply.

Here is my observations. I have download the boundary files from here: http://parlvid.mysociety.org:81/os/

When I re-project boundaries there no shift. But when i used any layer from the tile, it is shifted.

After this I have just compare the projection information of both layers.

Boundary file have this projection info:

Projection: Transverse_Mercator false_easting: 400000,000000 false_northing: -100000,000000 central_meridian: -2,000000 scale_factor: 0,999601 latitude_of_origin: 49,000000 Linear Unit: METER (1,000000)

Geographic Coordinate System: unnamed Angular Unit: degree (0,017453292519943299) Prime Meridian: Greenwich (0,000000000000000000) Datum: D_OSGB_1936 Spheroid: Airy - 1848 Semimajor Axis: 6377563,000000000000000000 Semiminor Axis: 6356256,161000000300000000 Inverse Flattening: 299,319997677743000000

and in tile layer for example NaturalFeature_Area have following projection info:

Projection: Transverse_Mercator False_Easting: 400000,000000 False_Northing: -100000,000000 Central_Meridian: -2,000000 Scale_Factor: 0,999601 Latitude_Of_Origin: 49,000000 Linear Unit: Meter (1,000000)

Geographic Coordinate System: GCS_OSGB_1936 Angular Unit: Degree (0,017453292519943299) Prime Meridian: Greenwich (0,000000000000000000) Datum: D_OSGB_1936 Spheroid: Airy_1830 Semimajor Axis: 6377563,395999999700000000 Semiminor Axis: 6356256,909237285100000000 Inverse Flattening: 299,324964599999990000

The bold lines are changed in both layers.

Any idea which one is correct?