Lostpanther

Hi guys. I am a newbie, i passed my cbt test two weeks. I would love some advice on a parking fine i just received. The insult here is that it is a free car park in a shopping retail park! But its a private car parking company. I parked in a section that was outlined on the ground in white, it was a large triangular shape. I was far from any lanes and pedestrian pathways. This caught me off gaurd as it appears the opposite ruling to public parking. I rode in and did not check parking signage as i did not seevit andit felt to distracting to look, so it appears I wrongly assumed parking off piste would be allowed. I shall atach a photo for you guys to see. Should i could contest this? If you think i have a chance what tactics should i employ? Thank you to anyone taking the time to reply with your advice and experience. Kind regards Matt