Taper Relief, Business/Non Business

If a property is sold of which the ground floor was a business premises the owner ran, and the first floor was a flat let for residential purposes. How do I go about determining the split for taper purposes? I need to determine what proportion to claim business taper and what proportion to claim non-business taper.
Do I need the advice of an expert to determine this, or the estate agent who disposed of the property?
Paul