42 chainring

[limey]

I fitted a black 105 5800 compact chainring set (50/33) when I built my new bike. I prefer the 50 chainring cos, on our flat terrain, I can push it, but I can't push a 52 0r 53.  However, I would like to change the 33 inner ring for a 42.  I have found some rings with the same PCD, but they all seem to have some qualifications which I don't entirely follow.  Is there a straight forward way to do what I want?