The only way that clause could be changed would be for you and the landlord to agree to the change. The "manager" may or may not actually be the landlord of your lease - you can ask your conveyancer about this.
If the landlord agrees then you can execute a deed of variation with them either removing or modifying the clause. If the landlord does not agree then you are stuck. There may also be covenants either in respect of the use of the flat itself or the communal areas that stipulate that you shall not cause a nuisance. If lots of people coming and going constitutes a nuisance then the lesees of the other flats could enforce this against you.
You would also have to speak to the local authority about your use of the property as well to ensure that you are not in breach of planning law.
Ask you conveyancer about the identity of the landlord and make enquiries regarding whether he would consent to such an alteration. If he does not consent and you start using it for your purposes then he can enforce the clause against you.
At the same time as your conveyancer about clauses relating to nuisance and if the same are contained in the lease (which they almost certainly will be) then you should check the other flat owners attitude towards your intended use.
The landlord and the other flat owners are the key here, not the vendor.
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