If a Silence looks at a Weeping Angel, does the Angel move forward, or does it forget it saw the Silence?

As long as the Silence looked at it, the Angel would be frozen. If the Silence looked away or blinked, the Angel could get it - as long as it could still see the Silence. If the Silence got out of sight before the Angel could reach it, the Angel would then forget about it.

the start of a good capaldi story me thinks first mentioned in the digitalspy forum!

That would be a great episode.