SSAP 21 Accounting for Leases.
This accounting standard deals with how to handle leases, perhaps if do a search on google etc you might find what you are looking for?
Basically only Operational leases effect the PNL Account,
Finance Leases are dealt with in the BS, as short term ( value < 1 year) and long term ( value > 1 year) liabilities.
In your CT calculation lease payments (less VAT) are an ADDBACk before you arrive at your Case I income figure, and as far as I remember you calculate Cap Allow as you would with any other asset.
hope this was of some help!
Snorkie