Having manage to get a faint blurred trace on my previous attempt at fixing this oscilloscope I figured on trying to increase the displays intensity and focus.
However I found even the faint trace to be very intermittent. I tentatively measured the heater voltage and it was getting the required voltage. The voltage rails on the oscilloscope are huge and would blow my cheap old multi-meter apart in an instant. I was shapely reminded of that when I slipped and gave a finger a quite zap, with a audible crack as something discharged into it.
With the power off I closely examined the PCB around the area of the CRT outputs, and I discovered what I believe to be the original fault. It looked to have been a manufacturing fault. A link wire that had been slightly bent over had missed the flow of the solder completely. Occasionally making enough contact with the bare copper pad and thus working from time to time. This is the most likely why the selector had been abused, trying to get a trace.
I had to solder it as best I could as I have not yet managed to find a way to remove this PCB without removing the transformer wiring.