Hi Gavin Anderson,
You would use the sum paid for the property as the purchase price and not the market value, as this sale was at 'arms length'.
Arms length is a term that means the there is no relation, by blood, adoption, marriage or two or more people who are living together, to the seller.
CG14550 advises that market value can be used where the transaction is not at arms length.
CG14550 - Market value rule: acquisition no disposal: disposal no acquisition
Thank you.