How to calculate a gain for SAYE scheme?

Hi, I have two SAYE schemes maturing this year and I'm unsure what the baseline is to calculate the gain for CGT. Is it the option price of the shares or the market value at the time of buying them? As an example, I've saved £100 per month for 60 months = £6000 savings. The option price was £20 per share so therefore 300 shares available. The current share price is £65 per share, so total value £65 x 300 = £19500. If I buy and sell the shares immediately is the gain: Using the option price: £19500 - £6000 = £13500 OR using market value at time of buying £19500 - £19500 = £0 (may be small variance if the price changes during the selling process) Any help with this would be appreciated.

Hi Iainw,
It is the option price.
Thank you.