qgis - OSGB36 to WGS84 reprojection 'error'

First off, I am a recent convert to QGIS and am greatly impressed with its ease of use and speed compared with the ESRI products with which I am more familiar - my congratulations and thanks to its developers (including the plug-ins).

I have surveyed (and corrected) DGPS (vector point) data collected to OSGB36 (British National Grid); these positions match acceptably well with the OSGB36 map layers to which I have access when imported as CSV and converted to SHP in QGIS, projected using EPSG:27700 (~1m error, although error could be in map layer which is suspect).

I'm attempting a four-stage export from QGISv1.7.4 to Google SketchUp (SU) via Google Earth (GE), as follows:

stage 1: In QGIS, Save as../KML with CRS set to WGS84 EPSG:4326 (or ETRS89);
stage 2: Open KML in GE;

stage 3: Save as KMZ;
stage 4: Open in SU.

Unfortunately, the exported WGS84 positions in the KML file are in error by ~8m - I've checked them against OSTN02 conversion using tool on OS website , I've also checked GE projection (by eye: converting GE lat/long to OSGB36 and comparing with OSGB36 vector map layer). (SU fails to open the KMZ file as well, but that's not a QGIS issue, I imagine - any hints welcome...).

So the evidence seems to point towards an error in the transformation from OSGB36 to WGS84 in stage 1, i.e. with QGISv1.7.4.

Any suggestions as to where the fault in my process might lie, or to an alternative approach, gratefully received...

thanks - JS