MI-L Errors in co-ordinates.

[John Eyre]

I am plotting points on an OS base map. When I click on the points the eastings 
and nothings are displayed correctly but when I transfer the co-ordinates to a 
browser using the centroid(x) and centroid(y) functions, there is a systematic 
error in the returned figures. The error is 100m in both co-ordinates. I would 
appreciate help from anyone who explain what I'm doing wrong.
John Eyre