If you sell the property in October 2017 for £300,000, you will make a gain of £100,000 (£300,000 - £50,000 (50% 1989 purchase price) - £150,000 (50% 2017 probate value)). You can also claim your share of the costs of buying and selling the property against the disposal proceeds (legal fees, stamp duty, survey fees, selling agent fees).
DISPOSAL OCTOBER 2017
Exempt gain: £40,708 (£100,000 / 339 x 138 (120 months of occupation by you + last 18 months of ownership)
Non-exempt gain: £59,292 (£100,000 / 339 x 201 (219 months of non-occupation by you - last 18 months of ownership)
Gross Non-Exempt Gain: £59,292
Annual CGT Exemption: £11,300
Net Taxable Gain: £47,992
CGT is charged at 18% or 28% or a combination of the two rates depending on the level of your income in the tax year of disposal on residential property gains. Take a look here for further information.
I hope this helps but let me know if you have any further questions.