JPB

I have owned a property for 30 years (bought for £25k), after owning the property for approx 5 years I remorgaged (an extra £35k) to have some work done on it. For the past 15 years the property has been rented out to a long term tenant who has recently moved out. Do I have to pay CGT on any profit above the original purchase price or is it something else? I remember seeing something about using the value of the property in 2015? I would really like to sell but if the CGT is on the original price I don't think the sale price will cover the remaining mortgage and the CGT 😕 Any thoughts would be much appreciated.