Well Chaps, many thanks for all the suggestions - what passes for my brain has been mulling them over and has finally come up with something.
Simon, I was rather dismissive of your S-shaped idea, but what I have now tested is very similar - the L -shape!
A few months ago a little art shop in my local town closed down, and sold it's stock off cheaply. I bought some big bottles of Liquitex Concentrated Artists Color acrylics in burnt umber and cadmium orange (great for making my rust, with the magic ingredient) neutral grey and cadmium red, some self-adhesive lead strip, used for making faux-stained glass (and great for fast fixing of extra weight on wagons), and some aluminium wire. This last item comes in a big roll and is 2mm diameter. It's easy to bend to the shape you want, and is exactly the right size to fit in the tipping arm fixings put on the ends of my wagons. I bent up an experimental L-shape and re-used the old tipping gantry to try it out - and it works perfectly, doesn't stick out so far, and the gantry doesn't foul any part of Didi's anatomy!
The prototype L-shape tipper arm in place.
The next step is to install a new tipping gantry, made from the alu wire, and 3 more L-shapes.
As for getting the tipper bodies down again, I am trying your magnetic method, Steve. You may just be able to make out a little washer on the end of the tipper arm. This is made of steel, and I'm experimenting with various magnets to get it to pull down on the tipping arm and so right the body. I may need a more powerful magnet, or a bigger washer, but the concept is good.
Watch this space!