So now it was time to start methodically testing the voltages in the amp with a multimeter to see which bit I'd cocked up.
Based somewhat on the testing plans over at Amp Maker and Tube Depot I came up with a diagram and some typical voltages that I should expect at each test point.
With the black multimeter lead grounded (either at the ground bus or on the chassis), with all valves installed and both switches on, the expected voltages are:
F - 345v
H - 320v
A - 1.4v
B - 83v
C - 3.4v
D - 11v
And removing the black from the ground:
E-D - 10.5v