CGT on Option

I recently entered into an option over some agricultural land. The developer will pay me a fee now and then a second fee at the point they decide to exercise the option and buy the land from me. I have received the first payment but do not know how/when to report that and pay the tax due. I have looked online, but nothing seems to apply to non residential property and/or options.

Hi SimonV,
There is this guidance:
Stamp Duty Land Tax - Rates for non-residential and mixed land and property.
And
Opting to tax land and buildings (VAT Notice 742A)
Also:
Capital Gains Tax: what you pay it on, rates and allowances
​​​​​​​Thank you.