Owning a property as joint tenants means that your father's share of the property will come to you automatically as you both owned the whole property. You will need to consult the solicitor as your father's will wanted it split between you and your brother. Take a look here
for information on the rules around joint tenants and tenants in common.
If you owned a third of the property as a joint tenant February 1984, your original one-third share would have a base cost of £10,000, When your mother died in 2004, I'm assuming you acquired half of her one-third share with a base cost of £27,500, one-sixth of the value of the property at that time. You then owned half the property for CGT purposes as did your father. Your half-share was worth £192,500 when your father died in March 2015 as was his.
The probate value of your late father's share for Inheritance Tax purposes is £192,500 and that will pass to you or you and your brother as the base cost in proportion to your respective shares of your father's half-share depending on the legal situation. Either you will take full ownership of the property or it will be put into your name and your brother's names. You already own half so in the latter scenario, he will own a quarter of the property and you will own three-quarters. This will need to be thrashed out by the solicitor as I stated earlier because of the joint tenancy.
My own view is that you are the new owner of the whole property and you will need to make a gift to your brother if you wish to honour your father's will. That would probably be best done by selling the property and giving him some of the cash raised.
Of your gain, you will qualify for exemption from Capital Gains Tax for about 6.5 years worth of the gain (about 5 years due to your occupation of the property plus the last 18 months of ownership) out of 31 years of ownership. It's normal to calculate the exempt and taxable parts of the gain using months as opposed to years.
If it is decided that the property has to come to you because of the joint tenancy, then your base cost will be £230,000 (£10,000 + £27,500 + £192,500). If you sell it for £385,000, you will make a gain of £155,000 of which £32,500 will be exempt from CGT. Of the balance of £122,500, the first £11,100 will be exempt from CGT due to the annual CGT exemption leaving you with a net taxable gain of £111,400.
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 tax year that the property was disposed of. If the sale occurs in 2014/15, one of the following scenarios will apply:
1 If the sum of your income and the net taxable gain in 2015/16 is £42,385 or less, then all the taxable gain will be charged to CGT at 18%.
2 If your income alone in 2015/16 is £42,385 or more in 2015/16, then all the taxable gain will be charged to CGT at 28%.
3 If your income alone in 2015/16 is less than £42,385 but greater than £42,385 when the net taxable gain is added, then part of the net taxable gain will be charged to CGT at 18% and part at 28%.
I hope this helps but let me know if you have any further questions.