Although this sounds a lot like bluster, you may have some liability. You do not have control over the alleged abandondoned vehicles and cant be held liable for any resultant shortage of spaces. Where you might be liable is if they can successfully argue that the wording of the advert and contract clause implies that a space WILL be available. That might facilitate action against you under consumer rights and breach of contract. You should consult a good solicitor at the earliest opportunity. I wouldnt suggest you reply until youve done that.