Retired2UK

I have recently moved back to UK and have now sold my house in UK and I am not sure of the position for Capital Gains Tax. I bought the property when I lived in the UK as my only and main residence and lived in it before I moved overseas in 2006. I lived and worked overseas for 18 years during which time I rented the property out. I have come back to UK to retire and have moved back into the Property as my main residence. I am now in the process of selling the property as I wish to downsize, but do not know what the Capital Gains position is: 1) Since the property was my main residence before I left UK and when I moved back to UK, is there a Capital Gains Tax to pay? Is 18 years too long to be considered "deemed absence" ? 2) If there is Capital Gains tax to pay, what date do I use to calculate the Capital Gain from? Is it the date when I moved overseas (2006) or is it 2015, the date at which overseas residents became liable for Capital Gains on UK residential property?