Well, you were on the money Nino .

I eliminated the CPU board by testing all row/column combinations using the jumper wire method .

On closer inspection the common wire on one of the trough switches was hanging by a thread .. fixed that but still have the trough switch fault .
Have found some cold solder joints on the  NC connection on a number of switches on the green/black column which has improved the trough switch issue but not fixed it as yet . Also reflowing the NC connections on all the blue/white row , but need to get to switch 36 (left ramp enter) as yet.

Reflowed the NC connections on switch 51 and that has fixed the switch 51 and 61 errors 

As usual I was over thinking the problem .. seems like its all due to cold solder joints .

What i noticed is when measuring the NC switch connection the resistance was greater than 1 ohm .. around 1.4 - 1.5 which is high enough to be causing problems .