Many thanks to everyone for their help and suggestions - I now have a working game!!! It was an incorrectly aligned ball count stepper unit coupled with a loose grub screw that made what I thought initially to be a fix, a little more inconsistant. It's all working correctly now, and I cleaned the contacts on the "Ball in Play" lights and changed the bulbs and they are all working now too (although they are very dim - does anyone have any suggestions for brighter bulbs (I replaced the originals ones with #47)).
Thanks again everyone,
Sarah