Found the problem, it was visible in the photos I posted - the sear was not properly fitted to the pistol, and was not able to sufficiently engage the hammer. Just had to stone the bottom of the sear a bit to allow it to drop to the sear stop - you can see how much space there was before. I agree that the disconnector/sear arm spring enagement looks a bit flakey, but it doesn't come off and appears to do its job.
The pistol works fine now.
