TL/DR: Ball count zero switch was closed when it should have been open.
The long version for anyone in a similar predicament who finds this in a Google search:
With everyone's help on here I've finally solved the problem. For future reference let me go through the issue and the steps I took to check everything.
The game plays fine on single player and scores and counts the balls etc perfectly. Pressing the credit button to add another player simply reset the whole machine.
I started at the coin unit which should add players and then worked my way backwards.
Going through the manual and schematic I can see that the coin unit adds players through the coin step up solonoid. I manually activated the linkage and confirmed it added players. Then I checked the coil itself. I checked the resistance across the lugs of the coil using a multimeter confirming the coil was good. To ensure it wasn't the sleeve that was binding I jumped the supply from the 6v transformer to the power lug on the coil and it fired. So, I know he mechanism of the coin unit and the coils are good.
Next, do the same thing for the player unit in the backbox. Again, manually advancing the unit correctly selected the next player. No need to check these coils etc because I confirmed it correctly set to the next player when playing a game so I know this works fine.
Whenever I press the credit button I get a full reset with power going to the coin unit reset.
Time to check the reset relay. I confirmed that all the contacts were in the correct position and that the contacts supplying power to the coin unit step up were clean.
So now I know the reset relay is functioning correctly, however it's being activated every time the credit button is pressed. This is activated by the game over relay so time to check that. This relay is a latch / trip relay. What this means is that the first credit powers the relay which latches on. To reset this relay a separate trip coil is activated which unlatches it.
I checked the position of the switches and cleaned contacts and confirmed that it's correctly latching when adding the first credit. The issue is that subsequent credits are activating the trip, resetting it. Upon the reset of the game over relay it activated the reset relay, which routed power to the coin reset. Aha! Here is the issue. The game over relay shouldn't be fully reset with each additional credit. Something is activating the trip.
The schematic and the manual show that if the player unit zero switch or the ball count zero switch are closed then it will route the power pulse to the game over relay.
Using a long wooden kebab stick I held open the ball count zero switch and pressed the credit button and hey presto, additional players are added!
So, simple adjustment to ensure the zero switch is open and problem solved.
Thank for all the help, it's greatly appreciated.