Lease breach by renting occupant

[YourRealFish]

Some neighbours are renting a flat next door and are breaching the lease by blocking our access. The renting occupants don't care and will not stop causing the obstruction. Who would have a letter of claim against them, would it be the occupants in breach or the flat owner (leaseholder)?

Just tell the landlord to sort it out, no need to immediately jump to 'letters of claim'. Ultimately it's up to them to ensure the tenant meets the obligations of the lease.

Sorry, I should have mentioned that the managing agent contacted the owner regarding the issue. The flat owner said they were unhappy that they had not been approached by the complainant directly to discuss (I don't have their contact details). The owner/leaseholder was more concerned about losing rental income than our complaint and invited us to talk to them. I left it with the managing agent to reiterate the access issue (with photo) and demand nothing short of the obstruction stopping. There is nothing more to discuss, hence I'm wondering about the next step if they continue not responding to the agent (it has been three weeks).

If there's a managing agent send a 'letter before claim' to the owner via them.

Letter before claim sets out the breach, what you need them to do, by what date and if it's not forthcoming you will take legal action.

Thank you so mcuh. I'll see how the agent handles it and this may be the next stage. Thank you for your help!

You haven said what the nature of the obstruction is.
If it is something in a communal area, I would be more concerned that it may breach the fire regulations.

If this is the case, I would pursue the managing agent in writing over the fire evacuation risk. Also worth checking if a communal area obstruction would affect the building insurance