Never refer to ‘appeal’; you are disputing an invoice.
Do not ignore correspondence from them.
Now for what you should do:
– identify the physical boundaries which define the area they ‘manage’ on behalf of decathlon
– if you parked outside this area, it is not their concern
– find name of decathlon uk md and write to him/her personally, and politely, outlining what happened; state you are regular customer (you are, aren’t you?); motorbike taking up little space and parked considerately; not depriving other customer(s) of parking space; the invoice is disproprionate to any economic loss which decathlon may have incurred
– was there a clearly marked and signed motorbike parking area
– the actions of their contracted parking management company have soured your opinion of decathlon; a well liked retailer but this incident will cause you to think carefully about whether you will continue to shop with them.
Be polite and factual.
Good luck.