After drawing up a list of every component in the -8v rail, I set about testing them all.
As R1565 (8K) was being removed it revealed itself to be broken in two. So that is some kind of progress, and I have temporarily installed a 10k which took the -8v rail down to -6.09v
By switching over op-amps U1554 and U1524 there were clear differences on the -8v, +5v and +15v rails, so therein lies another part to be ordered.
If I select the X10 Mag, a small (1") sweep does now appear, but without any vertical deflection.
I was trying to attach the power section of the schematic here