My opinion only, but if the items you have seen, Tony, have polished bases, then I would say they are the Czech Inwald items. A second factor to back up my feeling, is that the Clayton Mayers address on the ad you show is the earlier address of 28 Gray's Inn Road. At that point they were importing a lot of Czech glass, and Inwald's glass was being shown in the trade ads. The later ads for Clayton Mayers (where they state "Jacobean tumblers are British made") was when they were at their North Circular Road address.
If the base on those items has a mirror shiny polish/grind, then I'll stake a slice of carrot cake that they are by Inwald.