Well, I think I have discovered what my problem is
Yesterday was the first dry day for a whole week and I just
HAD to get out there and mow at least some of my (waist high) grass.
While doing that I noticed that the wild montbretia that is along the bottom of my lawn is also looking sickly ... just about dead on one side but still fresh and green on the other side. Also I noticed that a weigelia has also some similarly affected leaves, all about 100 yards away from my veg garden.
Then it clicked, they are all suffering from rust disease.
My veg garden wind break has a small gap between the end of the garage and the first hawthorn tree and the other side is bracken, amongst other things. The area of veg garden destruction points back to that gap, hence my first thoughts of wind born problem. A quick look over the back revealed .... dead bracken, suffering from rust, which spread their spores half way across my garden back in April/May, which then incubated for 6 weeks and spread at 90 degrees to the other side (my 'L' shape), and I presume is now incubating again.
There isn't much literature on general rust diseases on the 'net except specialist sites for specific crops, but it appears I have a variety that starts off on bracken and infects peas, broad beans, red cabbage, montbretia and to a lesser extent potatoes, beetroot & weigelia, but doesn't affect sprouts, leeks, lettuce, summer cabbage & onions. The one thing I did find out about rust is it is virulent when temperatures are between 16c and 21c and there is a lot of humidity about, which just about describes this drizzly summer to a tee.
I am relieved that I have sussed the problem and rather pleased that it is a disease, as it means that I probably won't suffer from it next year. It apparently has a very complicated life cycle which means it only appears once every few years and only then when conditions are favourable and in my case will only spread when the wind is in the north when it spawns.
There were a few weeks just recently when I really thought that my veg garden would have to be abandoned, but hopefully that's not the case.
Phew ...