Ask an Property Solicitor. Get an Answer ASAP.
I am sorry to hear about the situation.
The landlord can assist you in this circumstances because it appears your rights to the peaceful enjoyment of the property is and likley will be interrupted.
Do you understand what I have stated if yes I will proceed to the next step?
In other words there is likely a clause within your tenancy agreement which prohibits such actions therefore your landlord could rely on such a clause to evict the tenant in question. I suggest you contact your landlord and relay the circumstances of this matter. Ensure you inform the landlord your right to the peaceful enjoyment of the property is and will be compromised. Therefore he should consider taking steps to evict the tenant.
I hope I have clarified the position if not please let me know.
However if I have clarified the position please before you leave JA today please rate the assistance provided by clicking on the 5 stars button at the top of your screen as this will help improve the service given in future.