I'll give you an answer based on what I have but the cost of the plot will have to be divided between the two properties.
Whether you sell the properties together or separately, the gains will be calculated individually.
Assuming that you have lived in the converted chapel for the entire period of ownership or you moved in within one year of buying the plot and you sell it within 18 months of moving out, then the gain on that property will be exempt from CGT.
Assuming the property that has been let will never have been your main home before you sell it, then the whole gain will be taxable. The gain will be divided between the owners for tax purposes.
There are two rates of Capital Gains Tax, 18% and 28%. The rate or combination of rates that you will pay will be dependent on the level of your income in the tax year you sell the property. Assuming that it is sold in the current tax year, the first £11,100 of your respective shares of the gain will be tax free and one of the following scenarios will apply to each of you:
1 If your income including the taxable gain in 2015/16 is £42,385 or less, then all the taxable gain will be taxed at 18%.
2 If your income excluding the taxable gain in 2015/16 is £42,385 or more, then all the taxable gain will be taxed at 28%.
3 If your income excluding the taxable gain in 2015/16 is less than £42,385 but more than £42,385 when the taxable gain is added, then part of the taxable gain will be taxed at 18% and part will be taxed at 28%.
Take a look at HS283
for more information on the main residence and CGT.
I hope this helps but let me know if you have any further questions.