http://www.legislation.gov.uk/id/asp/2013/11/schedule/19/enacted

SCHEDULE 19Leases

PART 2Amount of tax chargeable: rent

6Net present value

The net present value (NPV) of the rent payable over the term of a lease is calculated by applying the following formula—

NPV = \sum_{i = 1}^{n} \frac{r_{i}}{{(1 + T)}^{i}}

where—

r_i is the rent payable in respect of year i,
i is the first, second, third etc. year of the term of the lease,
n is the term of the lease, and
T is the temporal discount rate (see paragraph 7).