As much as i do agree that bikers think they own the roads and im not surprised at the accidents ive witnessed over the years as a car driver, (ive also had a few near misses due to stupid bikers) your father did not signal, so it doesnt matter what the biker did, that becomes irrelevant, he also wasnt aware the biker was there...why not...if he was close enough to hit the biker then your father wasnt using his mirrors, nor did he check his surroundings before moving off, you yourself can contact DVLA and tell them of your concerns, it sounds like your father wont give up his licence without a fight, but hes just proved hes a danger to other road users, if he pleads not guilty and is found guilty then maybe the courts will order him to retake his test, these days driving licenses are only issued to the age of 75 so hes long over due...my advice...let him plead not guilty, because theres no doubt he is guilty, let the court punish him...but also get in touch with the DVLA as next time he may kill someone..goodluck