Follow Up: I've replaced VT4 with a new BF194 and still the issue with the retrace lines remains.
I've measured the voltage of the transistor with my DMM, 10.6v on the Collector and 0.7v on the Base. This appears to differ slightly from the schematic (12v Collector, 0.6v Base) but not enough to make me think this would cause an issue.
Any further feedback on where to go next would be greatly appreciated
