Once I completed it at medium (thought it was safer than hard) I went back and did it on hard with much success!
Having started a new profile I can see how the AI learns the circuit, they start by taking every corner exactly but eventually they cut the corners off a lot more.
AI stands for Artificial Intelligence but the AI isn't that intelligent, on a straight, they will fly into bombs or mines despite ample time to avoid them. STUPID!
Also, they will fire cannons as soon as they get them, aimlessly