It took a lot of fiddling to get the wiper blades to maintain contact with the windscreen through the full sweep. A lot of trial and error bending the arms but after about half an hour or so they were ok. Unfortunately this process was interrupted when the wipers jammed on the windscreen but the motor kept on pushing, and bent itself out of the flimsy bracket I originally made.
So, a beefier bracket was made out of L-section which screwed into the scuttle hoop from the side and underneath to make it massively more rigid. The first bracket only screwed from underneath and hence wobbled all over the place. The new bracket remained perfectly still and rigid. I had to replace the tube that the wiper cable runs in as that got all bent when the wipers jammed. The replacement stuff is a bit stiffer and seems noisier when it runs, even when packed full of grease.