Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
1 As the lease had less than 50 years to run when your company bought it, it is a wasting asset but as in your case a wasting asset can increase in value. The lease is a balance sheet item.
2 There may or may not be a capital gain on the sale of the lease. As it is a wasting asset, the amount of the allowable expenditure which can be used as the "cost" for CGT purposes reduces in line with the table here. Trading losses of the accounting period in which a capital gain in made can be relieved against the gain. Trading losses brought forward from an earlier period cannot be relieved against the gain. Look here for information on losses.
With regard to the leasehold improvements, the cost of some of these may have been claimed as revenue expenditure or capitalised and claimed as capital allowances. See section 7 here on enhancement expenditure. Where the enhancement expenditure is not reflected in the leases value when it is sold, it cannot be used in the calculation of the capital gain.
I hope this helps but let me know if you have any further questions.
Have you claimed capital allowances or a deduction from profits for any part of the £23,000 apart from depreciation?
I'm assuming that you would have added back the depreciation on the capitalised fixtures and fittings improvements so the loss on the fixtures and fittings is £18,000 (£20,000 - £15,000 - £23,000).