When I had the hybrid done a year ago (at Paddington police station which is only up the road, so it may be the same lot) it was some kind of liquid epoxy stuff painted through a template thing to make a dotted number. The number is put on a database. Fairly unobtrusive, near the BB. The police bloke who did it was a keen cyclist himself and seemed to know what he was doing.
There is also now the option of a chip being glued inside the seat tube, I understand.
Thanks Clarion, that would be fab if you could find it.