Advice required on this. Just had a bump exiting a roundabout, I was turning right in the correct lane, but there is a filter lane that comes off the roundabout from the left which due to the design of the roundabout gives priority to traffic coming onto the roundabout with having to giveway to their right (Lane1) the lane i was in is again due to its design takes you naturally into (Lane 2) after checking my mirror and putting the left hand indicator on, BANG a car had come up the inside on the left hand lane (Undertaking) to get past me, Never saw it with it 
being in my Blindspot !!! Its all on filmed on onboard CCTV. Boss says i am at fault technically, for turning into him,but is the car not in the wrong for undertaking? The guy in the car said he was reading the Caution Blind spot sign on the guard rails and thought he had better get past.
Well in my opinion, the guy saying he was reading the signage on your trailer rather than reading the road and backing off kind of says it all. I’d take that as an admission of fault as he told you he was reading signage instead of concentrating on driving as well as saying he wanted to get past you and tried to undertake and an incident occurred from it. . . That would be my take on it anyway but I could be wrong as I wasn’t there and don’t know the roundabout etc.