I've found a bit more info on an old thread from 2008, where somebody asked how to get to the underside of the PCB to solder it.
'disassembled' (Greg) responded:
"You have to remove the power tranny and faceplate. Once you do that you can lift the board enough to solder underneath."
I never thought of this! The PCB will still be wired to the valve sockets but I guess it's OK to bend all those wires to get to the underside. I'll have a go at this soon, but if anyone can chime in with more detail to reassure me I'd really appreciate it. Thanks.