I've read plenty of posts on this site regarding coordinate systems but none of them have helped solve this issue. I'm using Ordinance Survey Lat/Long coordinates for postcodes but a small number are being placed out at sea above Northern Wales (see image).
Surely if the coordinates were completely wrong, it would be more than a fraction of the total points that were incorrectly geolocated.
All layers are in OSGB 1936 / British National Grid (EPSG: 27700). Changing this to WGS 84 doesn't resolve the issue.