When I got my new (2nd hand) bike, there was no mention of a data tag.
When I was cleaning under the swinging arm, I noticed a very faint number. A UV light revealed the data tag.
I spoke to datatag, who confirmed that the bike had never been registered with them. This meant that I could register it free of charge, by producing a copy of my V5, which I did.
Datatag also sent me a couple of visible datatag stickers for the bike, along with the certificate. My insurers have also revised their price slightly to reflect the fact that the bike is data tagged.
That's a result!

DataTag