An old mill at the back of our houses has been converted into small units. When they built a car park they piled up earth round the edges of our gardens but they didn’t install any drainage. The result is that rainwater now comes through the mounds and floods our gardens. The landowners don’t seem to think it’s any concern of theirs.
Unless there’s something in your title deeds giving the car park owners the right to drain water on to your land (I think this is unlikely) you will probably be able to take them to court for “nuisance”. The owner may have to put in a drainage system for the car park and you may be entitled to compensation for the damage to your garden. I suggest you and your neighbours approach a solicitor jointly to present your case.