when shooting the ball thru the flying saucer and it goes under the playfield, the ball is not detected and sits under the playfield until it goes into ball search mode and then the ball is returned to the playfield, this happen every time. I think the problem is with the opto on the left hand popper assembly not detecting the ball, how do I test the opto to see if it's working.

When an opto fails very often it is the IR transmitter that dies - the receiver is normally OK. You can test the transmitter by using a digital camera (i.e. a mobile phone) - a working transmitter will glow purple on the camera preview screen (the camera "sees" a wider spectrum that the human eye and picks up IR). You can check a known working opto to see what you should be looking for, it's fairly obvious though.

If you find that you need to replace the transmitter, you can get replacements from nearly all the usual places - just buy the board, rather than ones with the plastic housing. If you're handy with a soldering iron, you can take the the blown IR transmitter off and replace it with one from Jaycar. I once fixed a friends table by scavenging an IR transmitter from an old TV remote - 18 months and 2,000 plays later, it's still good! ;)

Also the left hand popper seems to be too strong in that when it returns the ball to the left hand ramp on most occasions the ball will bounce off the ramp and back down the popper, is there any way of adjusting the strength of the popper ?

Check that you've got the right coil installed there - assuming you do, check that you've also got the right spring in the popper - a missing or weaker spring might not be providing enough resistance.

HTH