Thank you John and Clare & Peter, I appreciate your comments.
The incredible technology that is now available to amateur astronomers gives these ad hoc projects a greater chance of success than ever before, so it was well worth the effort.
I had 1st light with the RF 15-35mm F2.8L in our back garden on 22nd June, grabbing 5 x 30 sec shots of the Milky Way overhead, in the awful light pollution of Brisbane.
Lo and behold, I have now added Pluto to complete my classical solar system itinerary.
Cheers
Dennis