Calculation of lease surrender gain

[simplytax]

Where a lease agreement between a tenant and company landlord does not include a provision for payment to the landlord for early surrender or renunciation of the lease, as I understand it, the short term lease premium rules do not apply and the full payment is taxable as a chargeable gain.
However, if the building has numerous units available for lease and the surrender of the lease in question only relates to 1 unit, how is the cost attributable to the gain on the lease premium calculated?

The following basis of calculation would apply if the lease was for the entire building.

Cost of whole building x (Lease surrender value / Lease surrender value + Value of whole property after surrender of lease)

How is the above calculation amended where a payment is received by the landlord for the renunciation of a lease of 1 unit out of say 40 letting units in a building to arrive at the apportioned cost of the lease premium receipt?

Also, does a formal valuation have to be obtained or can the accounts value be used if it is noted as approximating market value?