Buy to let equity loan profit share fee

I took out a buy to let equity loan (against the value of one of my properties) 10 years ago which has just matured. The original value of the property was £180000, which allowed me to receive a lump sum of £36000 (20% of the values property) in return for a 40% share of any increase in the property value plus the return of the £36000 at maturity (10 years) The property has increased in value to £312000 so the profit share I have to pay to the company is £52800.(plus I repay the £36000) How do I treat this £52800 profit share fee? Do I offset it against capital gains when I sell the property in the future or is it treated as an expense I offset against my rental income from my portfolio in this years tax return? Thanks

Hi,
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Thank you.