The 'legal source' would have to have the canary address - so you could tell if it was used.

Unless somebody thinks this was a deliberate leak to poison the free data - in which case it would have it's own canary address

Why would there be a 'canary postal code' in the 'legal source?' At the very least, if it was there just in case the source was 'stolen' then they would have to tell people that legally purchased it to ignore that address. In that case, the information about the 'canary postal code' would be 'out there' anyways.

To stop me buying a legal copy and then immediately selling it as my own data. You don't have to tell the users that the data is there, simply generate a postcode that doesn't exist - then when a new postcode mapping product comes on the market, check for that postcode.

> then when a new postcode mapping product comes on the market, check for that postcode.

I realize that, but won't adding a 'bad' postal code possibly interfere with customers' usage of the data in the first place?

No because the customer would never use that bad address, none of their customers would ever use it as a home address for instance.

If they decided to post a letter to every address in the database then one of them would be undelivered - but that is probably not a problem given the efficency of the post service!

It's well-known that there are canaries in Ordnance Survey map data.