Okay, good news and bad news. Swapped opto boards and presto problem moved with the board so, left opto board is faulty. Still need to find replacement. Hard to find. Trying to avoid overseas suppliers oz dollar freight etc. And thanks heaps for all who helped. Much appreciated!  

You can change the faulty opto for this:
http://au.rs-online.com/web/p/slotted-optical-switches/7085538/

or buy a pair of new boards here:
http://www.johns-arcade.com/product-page/74eacd86-ba69-615b-5046-8db07256a378

with new actuators from here:
http://www.pinballspareparts.com.au/03-9001-2.html