'Just coming out of a 50, and into a 30'
Hmm well if it was a straightforward doing 51 in a 50, I would certainly go all the way with complaining, press, legal the lot. Speedos vary from car to car and you may be able to prove a difference of 1 mile an hour.
But the mention of '50 into a 30' leads me to believe you were doing 51 or more and had not slowed down before you got to the 30. I had a friend that argued there wasn't enough time to slow down for the lower limit. She of course lost in court.
Personally, I suspect you were doing much more than 50, then saw the 30 and were slowing down as a result and were doing 51 when you hit the 30. Or am I wrong?