Thanks Daniel, but that is not the one!  In saying that, mine has been ''modified'' and now I see what it looks like I can try and make mine mine work better

The one I am missing is under the playfield, to the right of the outhole switch.  The switch has the same wire colours as the one you have shown in the pic - I think it is to pick up when the two balls both push into the trough.

If you could get some photos I would be most greatful.

cheers
Ian