Further to your email and attached photos, as already advised this morning by Dee, you are not permitted to use the riser cupboard as it will interfere with the integratory of the Fire Precautions within the building.
Regards"
Clearly fire precautions wouldn't be serious enough, but as they're Fire Precautions, that's alright.
In the buildings I work in fire blocking is often certified by expensive contractors with method statements and insurance. I think it would be reasonable for a landlord to require that.
A response from the managing agent:
"Dear Mr Dammit,
Further to your email and attached photos, as already advised this morning by Dee, you are not permitted to use the riser cupboard as it will interfere with the integratory of the Fire Precautions within the building.
Regards"
Clearly fire precautions wouldn't be serious enough, but as they're Fire Precautions, that's alright.