I quickly swapped out the socketed custom IC's which are responsible for the track being displayed, it wasn't any of those :(
After a checking some more signals I previously looked at with my logic probe I moved to the area in the schematic labelled 'Roadway Memory and Adders'. From the descriptive name it could certainly be an area I needed to look at. Not long into checking signals, I came across signal 'Σ3' (Sigma 3) which is an output from a 74LS283. There are six of these 'Sigma' signals which are mixed in a 74LS298 with some other signals, the output from that LS298 are the address lines of the graphics ROM which contain the track data. Signal Σ3 was stuck always high in comparison to the other Sigma signals which were pulsing away. I removed the IC put in a new socket and with a new IC inserted we were making progress! In honesty I thought this was going to be the 'fix' for the board, but it was apparent there was now something else wrong as shown below.