Getting parking ticket in London city?

i had to take a partly disabled person to morfields eye hospital in london yesturday,i parked up on a vacant space down a narrow side street,i then walked across the road to the parking meta machine it cost �4 per hour,i then went back to the car for some change as i got to it i see a traffic warden checking the parked cars as i thought for pay and display tickets,as he approached me i walked to the pavement and said to him is that right it costs �4 per hour to park here,he replied and said yes,he did not look at my car for a display ticket as he knew i had just got there and carried on walking away,i then bought a �4 ticket and stuck it on my wind screen,when i came back to the car i hade a pennalty notice under my windscreen wiper,it said i had parked on a residents parking area,i trusted the warden to what he said,i thought i was all legal,i have now got to pay �40 in 14 days or �80 in 28 days,should i pay up and feel i have been done,or contess the offence,