Hi Rob,
Switch matrix issues can be good fun
As Nino said - you're well on the way - eliminating the CPU board is always a great start.
Me personally, I'd test continuity from J209 (pin 4) - white/yellow to the switch for that row closest to the back of the playfield - most likely 64 or 44 and work your way down from J209 on the playfield.
Also check it hasn't been pinched on anything (including headbox if its been down recently).
