The Lambert-made bicycles were sold as Lambert, Reg Harris and Viking. I've noticed that the headbadge on the OP's bike is almost exactly the same in design to the Viking badges in the link that Randyjawa provided. As the original Viking marque is listed as having gone out of production in 1967, so I'm wondering if the rights to the original company weren't bought out by the outfit that developed the Lambert. Given the latter's reputation, I can understand why an article on a classic British bicycle would be loathe to admit to the connection.

And yes, the Lambert, Reg Harris and Viking bikes were all identical. All had Lambert branded components, the same "aerospace tubing" sticker on the seat tube, etc. As to why they'd bother making the same bike under three different names, I've always assumed that this was a way to get more bikes into more dealerships without stepping on franchise territory limitations. Aka, once a dealer takes on a franchise, he's guaranteed a certain radius without a second dealer selling the same bike - legally, a Viking wasn't the same bike as a Lambert. I saw this done 40 years ago in Erie when you had Gitane, Roger Riviere, and Concorde dealers.The latter two bikes were bottom of the line Gitanes with different badging - and the lowest end Gitane you could buy was a one-step model above the bottom.