CGT on mortgaged property

I bought a property without a mortgage a few years ago. I then got the property revalued at its present rate which was more than the original purchase price and mortgaged it. Do I calculate my capital gains tax based on the original purchase price or do I calculate it based on the surveyors valuation price which was a few years after where I have a mortgage now?

Hi,

The capital gain is based on what you paid when originally purchased.

Thank you.