OK I’ve messed around, found a few not quite scaled objects (the bolt and nut from the add-on you suggested!), deleted vertices that seemed odd in the UV map from the model without it seeming to cause an issue. I’ve no idea what’s going on but have ended up with this which seems about right. Although the overall scale seems wrong as it doesn’t scale to fill the boundaries of the map. I guess I’d have to unwrap again and then average again to get that?
