As with speed limit signs (e.g. when entering an urban 20mph zone) you're expected to note a 'Controlled Zone' sign as you pass it
http://www.signsense.co.uk/images/whatwedo/HQ_controlled_zone.jpg
and remember that it remains in force until until you pass a sign telling you that you've reached the end of the zone
http://www.plymouth.gov.uk/parking-zoneends.jpg
In some city centres there can be well over a mile (or even several miles) between those signs.
So, if the signs actually exist, you've clearly got no basis upon which to lodge an appeal. However you might well be able to successfully challenge the ticket if you can show that there was no sign at the entry to the Controlled Zone.
The relevant local authority's website may well have a map showing the limits of the zone but, since you've not mentioned where the problem occurred, I can't provide a link.
Chris