Most of the Lentons I have seen for sale were not actually advertised as such and more often than not had been messed around with. I passed over one myself about a year ago because it was a Ladies frame, and less than £20 as well! I could see what it was by the lugwork, but it would have required quite a bit of sorting out. I'd have been tempted if it had been a 23" mens one though.
In terms of frame quality, my understanding is they are 531 plain gauge, so they will not be as light as a butted frame. A bit like a more recent Reynolds 500 cro-moly frame. Decent enough but not outstanding.