All I can add is that the inventory is just that - an inventory of what is in the property that belongs to the landlord and the condition, which I'm sure you operate.
eg, if there is a hole in a door, or damage to a kitchen work surface.
This is something that should be gone through in person by the agent with the tenants and agreed immediately before they move in precisely because the tenants may want to question something.
So they cannot include a locked area as part of the agreement onto an inventory because the tenants have no idea of what is stored there and cannot be held responsible for.
Stating that they were "told" carries no weight because as they can say, they were not told and so they could argue that they had the right to force the attic hatch open as they thought they had the right of access. They may not want to be doing anything like that, but equally, if this was not mentioned in the tenancy agreement so they may well have the right to force the latch as long as they make good any damage.
How do they know if's the landlord's stuff up there anyway and not something the agent is responsible for?
Is there a water tank up there - eg mains tank, old CH overflow tank? If so, they need access should there be any leaks or water damage.
You could also have a word with the local Fire Brigade which should do free checks on fire safety and see what they have to say about a locked area which the tenants have no access to and no idea of what is stored there.
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My body is an old warehouse full of declining storage, my mind is a dusty old reference library, strictly for members and archaeologists only
