A quick browse at the schematics led me to the section below:
/Score1 is the Video signal for the black Jet score and it was pulsing away on my logic probe. /Score2 was for the white Jet and that output (Pin 3 of C3) was stuck high. I checked the input signals and they all looked good so I piggybacked C3 with a new 74LS00 and the score came back.
I removed the bad chip, fitted a socket and popped the new 74LS00 in and everything is working again.