[Arran]

I'm beginning to wonder if the tube tester has some sort of defect like is out of adjustment, or someone played with the wiring, didn't you have a similar occurrence with some 6A8 tubes? If the tester was bought over fleabay you never know what someone has done to it, I've seen all kinds of strange things people have done to test equipment. The tube tester I have which is similar to a Hickock, a Canadian made Stark, has a scale setting switch needed for some tests, it can cause whacky readings if accidentally set wrong. Try testing some tubes out of a working radio and see what happens. If you can find someone with another tube tester and get a second opinion.
Regards
Arran