I seem to be getting a lot of wobble on my driver's side (maybe other side too but can't see that one as well) that seems to start just above 50 mph and stops at about 62 mph.

Not sure what I can do about it other then slow down or speed up to avoid those speeds.

I see in the book the brackets use the ball joint; mine just use 2 bolts holes of the cortina hubs so am thinking of adding this third support - anyone else done this?

Could be that your wheels need balancing,the speed you mention seems typical speed that vibrations occur if wheels are out of balance.

Rescued attachment wingstays2.jpg

Most have three M8 bolts that fit into the back of the Cortina hub.

Are you sure you only have 2?

Trying to prevent wheel wobble with only two fixing points is going to be pretty damn difficult when you consider the lever arm involved.....
However... I have the three M8 bolts but still get some wobble. I suspect the wheel balance too- the right wheel seems to wobble a lot more than the left!!! I know its not the wing support bracket flexing- STM had some problems with them cracking at the bends so I took up the option of the 5mm thick support arms!!! I can confirm (from bending them into the right shape) that it takes quite a lot of force to have any effect on that stuff (20mm*5mm bar!!)...
I'd suspect wheel balance though. A speed specific wobble suggests you've found the natural frequency of the arch and support bracket. Something like an out of balance wheel would be enough to kick off the harmonics... Why not try swapping the front tyres and see if it swaps sides

wheel balancing - hmm, didn't think that wold be it as new whels.tyres with balance wieght and don;t feel any bad vibes through the steering wheel.

will try swapping the tyres front/.back etc: but I reckon it is natural frequency ... but somehting mustvibrate to start it off so that can only be wheels.

re: the third bolt hole - that is be.ow the rear bolt hole so would mean the botton of the lever is bteer supported but not the top, it is the top where the problem is so don't reckong that will help too much.

My nearside front cycle wing flapped around more than the offside one then on Saturday the bracket broke while I was out for a blat. Classic fatigue failiure just above the weld where the flat bar joins the round bar stay. Its taken 1800 miles to break but in future I will be much less relaxed about it. I suspect you need to sort it out before it causes the bracket to fail.

Vents in the back of the wings can also help as it reduces the flutter effect.

Steve