I presume the Royal Mail will be using one of the regular updates of postcodes to place the marker (which will not be anything to do with us). These seem to be updated in a somewhat haphazard pattern: a little while ago we had a similar issue with new build flats near Mill Hill East station. One set of postcodes had been assigned and then replaced, and it was playing havoc with deliveries etc. Unfortunately although residents had been notified of the new postcodes, no-one else had!
I would double check what the findmyaddress site says for your postcode etc: this is pretty much as near as one can get to the best quality address data.
The other thing is if you live in a rural area the postcode marker may be some considerable distance from your house.
OpenStreetMap still only contains about 3-4 million UK addresses so it’s not something an official body would use for locating any address.