At present then the landlord should provide means for you to access your isolation even if it be by a phone call and him/her coming out with a key, if your business is hitting financial loss due to the present set up then I would be expressing that to the landlord to ensure your access or that the other neighbour is stopped from isolating your supply.
If you are been sold the building then the landlord has to either have an new supply fitted at his cost or reflect in the price of the property that it will require a new supply or alteration to the DNO cutout so your power is on the live side of the neighbours incoming supply thus no isolation point that can be turned off in the adjoining property.
Is the meter in your property a private meter (as I suspect - landlord owned) ?
I would discuss this concern with your landlord, personally I can't see him selling you the property without addressing the shared power set-up, he may have already made enquiries about having the supply divided, how this is achieved will require the DNO to inspect the set-up and quote the landlord.
Ps the Rented part was crucial information here and would have changed the advice others gave you, please try give all relevent info in your opening post, had I not asked then the advice you were given would be misguided and incorrect.
Also at 110 posts it would be nice to see you clicking the thanks option to show your appreciation every now and then when deserved