Share Incentive Plan Tax

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I am hoping someone can help. I recently left my old job where I put £150 a month into the company's share incentive plan scheme. For every four shares I bought I got one matching shares then I also got dividend shares. I bought the shares before paying income tax and NIC. It wasn't a PAYE share save scheme.

As I resigned I need to pay tax and NIC on shares that have been in the plan for less than 5 years. I received a letter saying how many shares the tax a NIC is due on and I agree with the figure so I am happy with this.

My question is on how the tax and NIC is calculated. I am a higher rate tax payer and it is not clear to me whether income tax and NIC has been calculated at the basic rate or at the higher rate. Is anyone aware of a calculator for doing this? I asked the company that adminsters the scheme but I am still not sure as the answer wasn't clear and I had to go somewhere so couldn't spend long on the call. I want to make sure I put money away if more tax is due.

Thanks for any help.