Tax on disposal of property

Other

I bought a property in London in 1975 which had a sitting tenant at the time but has been rented using AST since 1993 and although I have remortgaged it several times I am now considering my options

option 1
Refurbish and extend (which would increase the value considerably)and then possibly sell it or rent it again

option 2
sell the property now

I worry about the amount of capital gains tax that I would have to pay. Does the fact that I have owned the property for over 35 years make any difference? Does the old taper relief apply for the period the property was owned prior to the new rules which came in with the present government.
I would be very grateful for any advice