Quoted from ajfclark:

In the end I did replace C1, but that didn't fix the issue. I pulled the boards out to reflow things and they're a mess from previous repairs and mismatched LEDs. To keep the machine running while I repair the original trough boards, I bought a new set that mounts with the rubber grommets. Machine now plays 99% right.
The 1% is cannon related. Sometimes, especially during the initial portion of Borg multiball, the left cannon fails to fire. You hear the phaser noise so I assume the trigger works, but something else obviously isn't. In test it seems quite irregularly timed. I've never had a WPC so I don't know if that's normal:
The left cannon is also slower than the right, and both cannons test at startup, turning and firing the coil.
First port of call will be to disassemble the cannons and clean everything, but any tips on stuff to look out for would be great.