Just finished cleaning the lens and discovered why the camera was vignetting under some conditions. The lever that cocks the shutter was badly bent, as it looks like the camera was dropped at some point in the last 50 years. The lever has a limited amount of travel and when the shutter is released, it flies back to the home position. As the level was bent it was reaching the end of it's travel before it was in the home position, meaning that the shutter was never fully open. I've now bent it into the correct shape and the shutter works correctly now.
This web site was useful to show how to open up the lens board and get at the shutter: http://www.instantoptions.com/conversions/350/350.5.php