What can I do under property law if I have 2 carparks on my lease and the business next door's clients keep taking my space?
Just be aware that it may be a breach of your lease to "store" things (including vehicles) in the car parks. Where the difference between that and legitimate parking lays can be a source of contest if your landlord wants to be argumentative.
Clancy, that's not a sensible thing to say.
If you are putting "old non functioning motor scooters or motor bikes" in the car spaces, it will be pretty obvious what they are. If you have a clause in your lease which prohibits you from doing that sort thing, then your landlord may take issue with it.
Not will, but may. And it may not ultimately make a huge difference. However, if you're wanting to get the landlord onside for any potential argument against other tenants/landlords about rights to a car parking space and infringements on those right - then the last thing you should be doing is annoying your landlord.
I was simply raise the issue as something that the original poster may wish to take into account when making his decision on how he wants to proceed.