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So in your set, the fine tuning control would be used to peak the amplitude of the sound signal.
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Chris,
Sorry to quote myself, but there is a silver lining here.
I just realized while driving to work and thinking about your TV problems and how to help you with some practical advice, the fact that your set is not an intercarrier sound model can actually be used to help you fix the set.
In this design, when you feed the set's tuner in the usual way with the TV signal, there will (or should be) a position of the Tuner's fine tuning control where the sound is best. It might not be a particularly sharp peak but it should be obvious where to set it. So If you can set it there, what you really have passing from the tuner, via the first vision & sound common IF amplifier tube V8, then the sound IF amplifier stages and then to an FM detector, is just really what amounts to a standard FM radio which no doubt you are already familiar with.
So one approach in fault finding here, is to completely ignore for now any of the faulty vision functions of the set (rest of the video IF & amp stages , sync seps etc) turn the CRT brightness to zero.
Just concentrate on working on the sound receiver system and getting that right before anything else. And think of it as repairing a typical valve based FM radio. When its working the sound would be expected to be crystal clear with good volume.
So work your way systematically on testing all the way from the tuner, V8, V3, V4, V5,V6,V7 and try to get that working before moving to any residual fault in the vision system.
(The above method doesn't work well for the intercarrier sound take off method because the entire vision IF has to be set up properly for the sound IF to work.... but luckily this doesn't apply to your unique TV set )