If you sell the property in December 2016, by then you will have owned it for 314 months of which you will have lived in it for 18. As the property was your main home, you will be given the last 18 months of ownership as a tax free period so your tax free period will be 36 months. The balance of the gain will be taxable.
As far as the cost of the property for CGT purposes is concerned, that should comprise of the sum of one-third of £19,125, one-sixth of the value in 1998 when you Dad passed away and half of £130,000, the value when your Mum passed away. I will use £77,750 (£6,375 + £6,375 + £65,000) as the cost for CGT purposes. You can add to that the cost of buying and selling the property, £2,200, and the costs of any improvements. HMRC may ask for proof of any improvement costs that you claim.
Using what I have the gain will be £50,050 (£130,000 - £6,375 - £6,375 - £65,000 - £2,200). The sum of the gains for the period that you lived in the property and the last 18 months of ownership will be tax free. That accounts for £5,738 (£50,050/314 x 36). That leaves £44,312 (£50,050/314 x 278). The first £11,100 of that will be tax free due to the annual CGT exemption so you will be left with a net taxable gain of £33,212. As you are a higher rate taxpayer, you will pay CGT at 28% so your CGT liability will be £9,299.36, payable on 31 January 2018 assuming the property is sold by 5 April 2017.
If you can get a value for the property when your father died and verify some of the improvement costs, you will be able to reduce your CGT exposure.
I hope this helps but let me know if you have any further questions.