Page 1 of Smart parking

Having read a old thread about the above Cowboys would just like to say I just returned to find a PCN attached to my windscreen. I was on a industrial estate and away for about 20 minutes. Ticket states unauthorised parking yet the only sign I saw was a no overnight parking. They want £50 or £100 if not paid within 14 days. Am I right in thinking this is not enforceable ? Thanks