So do you think adjusting the rear band as you described to Jonboy would be my first approach? Seems the simplest way forward.
You can try that, I have had the adjusters work loose in the past, but if the adjustment goes up a long way then that is indicative of another problem, such as the servo housing cracked or loose on its mounting. As with jonboy, he went through the whole process and it turned out to be a sevo seal failure.
Mines a '72 RHD - what side of the tunnel will the adjuster be.
Driver's side. There's an access hole above the adjuster in the tunnel.