There is a general rule which states that you should turn in such a way as to pass behind the other aircraft but that theory is really and practically limited to to you seeing an aircraft on your right hand side and on a converging course a long way away.
In reality and as you have found you tend not to see the other aircraft until it is virtually on you and gut reaction takes in. Most Pilots will take a self preservation route and manoevre in whatever direction will take them away from the converging aircraft - climbing away is not generally an option so a steep bannked and propbably descending turn away from the threat is probably most people's reaction.
I had a similar occurance myself in August whilst flying steadily in a Sw'ly direction in the late afternoon / evening. Suddenly and without warning an aircraft appeared at my two o'clock blocking out the sun. I should have seen and I should have taken avoiding action because it was on my right hand side but I could not possibly have done so as the other aircraft was in effect a stationary target within the sun and it was only when it got so close that it became bigger than the sun did I see it. Gut reaction kicked in to get me away from the 'threat' and I did a violent left hand dive. I don't know if it was the 'right' thing to do - the options to do the 'right' thing become pretty limited at such close range.
You did OK - you avoided a mid air but I would suggest that you report the airprox anyway to help everyone else in the future. Remember there is no blame apportioned in Airprox reports.