Thanks for the additional information.
If you sell the property in December 2013 for £800,000 you will make a gain of £699,000 (£800,000 - £26,000 - £75,000). By that time you will have owned it for 356 months of which you will have lived in it for 257 and let it for 99.
The gain for the period the property was your main home is exempt from CGT as is the gain for the last 36 months of ownership. That accounts for £575,301 (£699,000 / 356 x 293). The remaining gain of £123,699 is that part of the letting period gain which is not covered by the last 36 months of ownership (£699,000 / 356 x 63).
As the property was both your main home and it was let you are entitled to letting relief which is the lesser of:
2 the sum of the main residence gain and the gain for the last 36 months of ownership of the property which is £575,301 and
3 the letting period gain of £123,699.
Letting relief of £40,000 will reduce the remaining gain of £123,699 to £83,699 and the annual CGT exemption of £10,900 will reduce it further to £72,799, the taxable gain.
There are two rates of CGT, 18% and 28%. The rate or combination of rates you will pay will be dependent on the level of your income in the tax year of disposal of the property. Assuming you sell the property in the 2013/14 tax year, one of the following scenarios will apply:
1 If your income in 2013/14 including the taxable gain is £41,450 or less then all the taxable gain will be taxed at 18%.
2 If your income in 2013/14 excluding the taxable gain is more than £41,450 then all the taxable gain will be taxed at 28%.
3 If your income in 2013/14 excluding the taxable gain is less than £41,450 but more than £41,450 when you include the taxable gain then part of it will be taxed at 18% and part at 28%.
Letting relief of up to £40,000 is available to each part owner of a property so long as they had an interest in the property during the period when it was let and it has been their main residence either in reality or by reason of marriage.
I hope this helps but let me know if you have any further questions.