I've had a few PCNs from Ashford before, never paid one though.
This is fairly clear-cut, as the terms and condition board makes no mention of permit holders, so there's no reason why you would look for permit holder signs. Here's some draft reps you can send when you get the Notice to Owner:
Dear Ashford Borough Council,
I contest liability for PCN AS09113666 on the grounds that the alleged contravention did not occur. The terms and conditions board which is required to convey the main requirements created by the off-street parking places order makes no mention whatsoever of any permit parking places, it simply indicates that payment must be made, vehicles must park wholly within a marked bay, and vehicles parked in a disabled persons' bay must display a blue badge.
I have complied with all of those requirements, and it follows that on this occasion a contravention did not occur.
Yours faithfully,
Don't expect them to accept, but I think they'd really struggle at the tribunal with this one.