Sp!ke, did you not check your expiry time on the ticket you purchased?
As has been said already, you are required to pay the hourly rate (£1.20 per hour or part thereof) until the evening rate kicks in, then you pay the evening rate. Harsh as it is to pay £1.20 for 6mins that's the way it is - you're effectively paying up to an hour for each £1.20.
You mentioned challenging this for no contravention. A contravention has occurred so that's not technically correct on your part.
You have the right to make an informal
challenge which is what I would do, explaining how you find the signage misleading. It is likely they will respond and state it is clear, but if you feel it isn't wait for your Notice to Owner to be served, and state again in your formal
representation that the signage is unclear in your mind.
At this stage you will have lost the right to the 50% discount on the charge amount to pay, but that is the regulations courtesy of the traffic management act 04.
If they reject further, you can
appeal to Tribunal, and state to an adjudicator who is independent of the authority, that you consider the sign misleading. If they agree with you (which to be fair they may well) then you will "win" and pay nothing. If Tribunal reject your appeal then you will have to pay the PCN.
Either way, ignore it and the charge increases by a further 50% (thanks tma'04 again) following the issue of a charge certificate. Continue to ignore and it's registered for warrant of recovery through certificated bailiffs.
PCN process is explained here:
ParkingEnforcement
And the tribunal stage here:
TMAAdjudicatorsHome
Go to pepipoo if you like, it's a reasonable justice fight site, but a lot of rubbish is posted there frankly; enter at your peril