"It is your duty to make sure that the third party was aware of your presence on the road, which he appears not have been, before attempting to pass him."
I don't know if this is law or not but it doesn't matter. Drivers struggle to indicate as it is, so if a driver is indicating it's pretty damn certain he's going to turn. Passing him as you did was stupid.
You keep telling yourself that.
http://www.roadsidelawyer.co.uk/questions/driver-stopped-in-middle-of-road-as-if-about-to-turn-right-but-as-i-passed-on-his-left-he-set-off-again-running-into-the-side-of-me-who-is-liable/?searchterm=nearside
"It is your duty to make sure that the third party was aware of your presence on the road, which he appears not have been, before attempting to pass him."
I don't know if this is law or not but it doesn't matter. Drivers struggle to indicate as it is, so if a driver is indicating it's pretty damn certain he's going to turn. Passing him as you did was stupid.
End of.