UK Landlord Forum / Life

Hi I am very new to this.  I am negotiating a rent review and my tenants agent says that as the occupation of the flat above the shop is restricted to the shop trader there is a convention in such circumstances that the rent attributable to this part of the accommodation under the principle lease should be at 50% of that which might be achieved for same accommodation if let on Assured Shorthold Tenancy. Is that true and is 50% reduction negotiable ?  thanks Colin