Your gain will be calculated by taking the value of the property as at 31 March 1982 and adding to that the cost of any improvements you have made to it since then. You deduct that total from the disposal proceeds of the property. You can also deduct the costs of purchase and sale (legal fees, stamp duty, survey fees, selling agent fees). That will leave you with a gain or a loss. If you have a gain, you deduct the annual CGT exemption of £11,100 to arrive at the net taxable gain.
There are two rates of CGT, 18% and 28%. The rate or combination of rates that you will pay will be dependent on the sum of your income and the net taxable gain in the tax year you sell the property. The most that can be taxed at 18% is £32,000 and that will be reduced by £1 for every £1 of income you have in excess of £11,000, the personal allowance. Any part of the gain not taxed at 18% will be taxed at 28%. Unless you have lived in the property since 31 March 1982, you won't be entitled to any main residence relief.
There is a calculator which will give you a rough guide to your CGT liability here. Let me know if you need any help using it.
I hope this helps but let me know if you have any further questions.
OK. Good luck with the sale.