Teaser 2549: A round trip

From The Sunday Times, 31st July 2011 [link]

I own a circular field with six trees on its perimeter. One day I started at one tree and walked straight to the next, continuing in this way around the perimeter from tree to tree until I returned to my starting point. In this way I found that the distances between consecutive trees were 12, 12, 19, 19, 33 and 33 metres.

What is the diameter of the field?