I have worked in motor claims before and, I am sorry to say, that you would be held principally responsible for the accident. Technically you have got to give way to all traffic on the major road... even if the are driving on the wrong side of the road!! It is not an uncommon type of accident and you cannot assume that the next vehicle will not slow down or change direction. You are correct in that the other driver should not have overtaken at a junction and, in defensive driving, such occurrences can be avoided by slowing down rather than overtaking. Had the other driver been 'tailgating' the car turning left he may have moved out to avoid collision. He would have run the risk of being charged with careless driving also but without direct evidence to that effect is likely not to be charged.
Jock