[Arran]

Russ;
  Did the 1L6 you tested give the same readings if you did not let the tube "cool" down first? Do you know if the reading dropped down, and stayed that way? I'm wondering if it might fluctuate, up and down, if left in the tester for an extended period. I'm also wondering if the loctal 1LA6 suffers from the same issue? The condition you describe, in the radio, does sound a bit like flaky carbon comp resistors, which is always worth investigating.
Regards
Arran