Update .. I've put the original CPU board back in with the driver board from Tri Zone .
Have played about 25 games with no issues (except I couldn't even beat the default high score of 550,000  )

Will need to continue playtesting (damn hard job but someone has to do it  ) as at this stage test results are inconclusive.

I am quietly confident that the sluggish right kicker was the cause of the solenoid fuse blowing as that fuse only protects 28V (ie playfield and cabinet solenoids) but we'll see.

Buggered if I can understand how a driver board fault could have anything to do with missing score display digits 

Thanks for all the help so far guys and stay tuned for updates

Cheers
Dave