Here is what you do.
1.Write to them and say you are the registered keeper of Nissan reg no. ABC 123D or whatever.
2. State that several people have access to the vehicle and you have been unable to ascertain who was responsible for parking the said car on the said date
3. Ask THEM to provide evidence of who was responsible for parking the car on the said date etc (this covers you in the unlikely event there is a CCTV camera or their parking attendant snapped you on camera. If they do provide the evidence, youu will have to stump up)
4. State in your letter that unless THEY can provide evidence of who was responsible for the parking contravention, you do not want to hear from them again
5. If they continue to write, ignore the letters.
6. UNDER NO CIRCUMSTANCES should you pay and then appeal - you will never see your money again.
Trust me, this strategy works.