Thanks for the suggestions. I have been told it's in mm, do you know how I could check?
That said I have just tried setiting the projection to WGS84 using a PRJ file and the following:
layer.FeatureSource.Projection = new Proj4Projection
InternalProjectionParametersString = Proj4Projection.ConvertPrjToProj4(prjDefinitiion),
ExternalProjectionParametersString = Proj4Projection.GetEpsgParametersString(27700)
where prjDefinition is the text read from a .PRJ file, like:
and still get the same exception if try to do a GetBoundingBox. So I'm thinking there is something else not quite right?
Oh and also, filepath and layername are not null.