I lashed together John's tester with a spare RS audio output TX and I get a lit dial bulb
The PL81 looks happy and does not seem to be overheating if I leave this rig running for a few minutes. Turning the horizontal hold which changes the line frequency also dims the bulb slightly with higher the frequency.
I'm going to triple check all my work around this end of the set - as you may have seen from other threads I have slipped on a few banana skins before!