Search Land Registry by postcode
The Land Registry publish a set of data each month (under the Open Government Licence) every house that was sold, its price and some other details. Since this data includes the address and postcode, we can match this data against OSM to help fill in some postcode gaps.
Type in a postcode (or postcode fragment like "CV4 8") into the box below, and you'll get a list of all the matching sold houses in the area. The site will attempt to match each of them up to an address in OSM. Use the tool to find incorrect OSM postcodes as well as ones that are missing entirely.