BNP Paribas is charging £220 subletting fee – for very little

One of the properties i manage have recently send out subletting consent forms and advised that the charge of £220.00 + VAT would be payable once the form was received.
The managing agents are BNP Paribas and the head lease states that they are allowed to charge a reasonable fee for subletting consent.
We have argued that this is not a reasonable fee and that the cost is exorbitant but they have replied with the following; “The fee is reasonable and will apply to the registration of the subletting and the work that goes into facilitating the review and administration process through to providing Landlord’s consent by way of a formal document”
Would we have to get solicitors involved to get this lowered as they refuse to negotiate at present.