So.. here are some more photos.
Originally this amp was made using a system of ground 'nodes' strung together to the bus running through the middle of the preamp board.
After I proved the amp would switch on and work, I decided to convert this to something closer to a star system, but one that still included nodes based on the ground connections of the filter capacitors. The valve wizard website has a great PDF on grounding. Imagine being called 'Merlin'? Pretty cool eh? I have a friend who has the surname Wizard. I say friend, I don't like him that much but he has a cool surname (He's alright actually... I guess).
Comparing the two photos you can see the divided ground bus. Eventually individual wires connect the filter caps to a 'star' ground.
The input jack has a small cap to ground attached to it, which creates a low resistance path to ground for radio goblins carried along the shield of the instrument cable.
Put short... this is all that 'special sauce' that stops the hum gremlins getting wet or being fed after midnight.
Some of those wires in the photos are still temporary, and will get fixed when the pots are upgraded to Clarostat 2Watt Conductive Plastic mil spec types. This keeps the other kinds of Gremlins as cute Mogwais.