Update:
Problem solved, it was the U/V tube that had suffered a partial-existence failure.
I bought a twin-light kit today (blue and U/V tube + power inverter). by a process of elimination, - trying the new tubes in the old power block, and the old tube in the new block, it's a dead red U/V tube.
My PC does look rather pretty now, along with the rest of the room from the blue glow
