It's a shame because, had the property been bought by 5 April 1988 and occupied by your mother in law by that date, you would probably have qualified relative relief which would have exempted the gain covered by the period that your mother in law lived in the property and the last 18 months (previously) three years of ownership.
Had the property been your main home at some point you would have qualified main residence relief for the period of occupation and last 18 months (previously three years) of ownership. If it had been let as well you would have qualified relief. Read about those reliefs in HS283 here
Unless the property has been your main home at some point during your ownership of it, I'm afraid that there are no reliefs other than being able to claim cost of improvements to the property, the buying and selling costs and the annual CGT exemption of £11,000. You cannot offset interest paid on the mortgage. That would have been deductible from the rental income had it been let.
If you sell the property by 5 April 2015 for £230,000 you will make a gain of £202,000. The first £11,000 of the gain will be exempt from Capital Gains Tax leaving you with a net taxable gain of £191,000.
There are two rates of CGT, 18% and 28%. The rate or combination of rates that you will pay will be dependent on the level of your income in the year you sell the property. Assuming you sell the property by 5 April 2015, one of the following scenarios will apply:
1 If your income in 2014/15 including the net taxable gain is £41,865 or less, then all the taxable gain will be charged to CGT at 18%.
2 If your income in 2014/15 excluding the net taxable gain is £41,865 or more, then all the taxable gain will be charged to CGT at 28%.
3 If your income in 2014/15 excluding the net taxable gain is £41,865 or less but more than £41,865 when you add the net taxable gain, then part of the net taxable gain will be charged to CGT at 18% and part at 28%.
If the property isn't already in joint names and you don't qualify residence relief or letting relief, you may be able to reduce the amount of the net taxable gain chargeable to CGT at 28% by putting the property into joint names before you sell it. You would need to look at your respective incomes before making such a move to avoid paying more tax even with the benefit of an additional CGT exemption.
I hope this helps but let me know if you have any further questions.