[Zelandeth]

Those caps were replaced because while they were identical to many others in the set, they were the only ones where the rubber bungs in the ends were bulging out, and sure enough when removed one of them turned out to be leaking slightly. As I had suitable spares handy, seemed sensible to change them.

The 0.08V there was relative to chassis. Measuring relative to k (pin 3), I get -4.2V and -3.8V. Cathode being positive wrt the control grid (and around 5V positive relative to the chassis).

I did try the lamp behind the boards trick, unfortunately they're just a bit too thick and opaque to actually be able to see the tracks through. You can see that there's a track THERE, but not quite where it goes.