Unfortunately, it seems to be valid, meaning that the rights outlined in the letter would be correct. You can still keep the gate there, however will need to allow access in accordance with the deed. However, they should not be using the land for parking overnight or extended periods of time, or anything which is not specifically permitted in the deed. If this is what they are doing, I would advise that you write to the solicitor in return and advisee the same.
I shall provide you with a helpful link that will assist you in finding a solicitor/representative near your local area. This will provide you with someone nearby your area who can assist you if required.
I hope this information proved helpful. You will find a local solicitor who deals with these matters on the law society webpage which is as follows;
https://www.lawsoc-ni.org/solicitors (N. Ireland)
Hopefully, I have answered your query in a way that is simple and easy to understand. If anything remains unclear, I will be more than happy to clarify it for you. In the meantime, thank you once again for using our services.
Should you require any further assistance on this matter, please do not hesitate to post a further questions for additional assistance.