The covenant on the land is not going to help you unless whoever has the benefit of the covenant is going to enforce it and for whatever reason, it seems they are not bothered.
However the method of enforcing it is the same whether it you which wants to stop them trespassing on your land or whether you are enforcing covenant.
They are clearly taking no notice of the agent and no notice of you. Perhaps they will take more notice of a letter from a solicitor threatening an application to court for an injunction to stop them parking on your land (trespass) and an application for legal costs.
I would put together a diary/chronology along with photographs of all the dates and incidences where they have denied it but you have photographic proof.
One solicitors letter and if they don’t then start to play ball, it’s an application to court for an injunction to stop them. You also asked the court to award costs against them.
Don’t waste time in copious correspondence.
You may find that a strongly worded letter from a solicitor threatening a court application and an application for costs, may focus the mind without actually the need to get to court.
Check your house insurance to see if you have legal expenses cover that would pay for the legal cost of taking this to court.
There is no reason why you could not DIY if you have experience of drafting legal proceedings but you really need the solicitors letter to try to avoid it if possible.
The problem for you, is that if you are not in the UK, you will be required to attend any trial hearing and you will not recover the costs of coming back from abroad.
The fact that you may have lost buyers is not a reasonably foreseeable consequence of the trespass.
There would be nothing at all to stop you putting a couple of parking posts to prevent this. Or gates or fences but to be honest you shouldn’t have to go to that trouble.
You potentially have a claim against the landlord of the next-door property for him allowing his tenants to cause nuisance but to be honest, why you would threaten that in the hope that he would have a word with his tenants, he is not one of the people I would be taking to court.
Can I clarify anything else for you?
I am happy to answer any specific points arising from this.
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