Is there any reason why forfeiture of their lease was not explored?. As it appears to me that they are in breach of the terms of the lease due to the interference. The First Tier Property Tribunal can make such a determination and is quicker and cheaper than the usual county court action.
I will proceed on the basis if the N16a injunction which you are asking about though.
So you can in this situation apply to the local county court for a civil injunction.
However, before you do this, the court would expect you to send the neighbour a cease and desist letter to warn them of your intentions unless they stop (and give them 7 days to stop their conduct). A template “cease and desist” letter is available at https://www.lawdepot.co.uk/
If you go to the search box in the top right corner of the site, it will bring up a list - you want the "general" letter.
You can use your own letter if you wish - it just has to make it clear this is a cease and desist letter, that you demand they stop their conduct, and that if they do not within 7 days you will apply to the court for the injunction and seek your costs.
That letter may well be enough to stop the conduct - as an injunction is a serious legal remedy. I know you probably don't want to send the letter and delay matters further but you would have to answer some difficult questions from the judge if you choose not to. As the judge will want to see you tried to avoid taking this very serious step by giving the neighbour a clear warning of the injunction as a consequence of their actions.
Assuming they continue, you will need to complete the application for the injunction and send to your county court with a fee of £308 payable to HMCTS (unless you have a low income/low savings and/or you are in receipt of benefits in which case you can get a fee exemption so you do not have to pay).
You will need to detail the history of the issues with the neighbour to paint a picture for the Judge as they will not know what this neighbour is like and what they have done to you unless you describe it.
If the injunction application is successful you can ask the court for a costs order to be paid by the neighbour.
The court will list a hearing where you attend and ask the District Judge for an order and the injunction. The application would be made under the tort of nuisance due to the interference.
If granted, the Order is then served on the individual concerned and they are bound by the court order - if they breach the order, they are in contempt of court and could well face a fine and a prison term. The order will say they cannot continue to inference as they have been doing. They are very effective given the consequences of breaching the terms of the injunction.
The cease and desist letter will hopefully work so you may not need to apply but if you then I recommend you go to the court in person with the forms to ensure they are correct - they are fairly complex so it would save time by you doing this - the court staff should check them before they issue the papers.
If you prefer a law firm to do the letter (and apply for the injunction if the letter has no effect) then try any of these firms they should be able to claim their costs from the neighbour so would be worth asking them to help you with this: