Premium bonds - any tax paid when cashing them in early

I have an elderly relative and they have some premium bonds which they wish to sort out. I understand they cannot transfer them but I would like to know that if they cash them in do they pay any income or capital gains tax on them? If so is it on the whole value including when they have won prizes or just a change in value (ignoring the value of the prize which they have just had reinvested in the bonds) thanks

Hi CLIVESMITH2001,
Premium bonds are free of capital gains tax, stamp duty and income tax and do not count towards your personal savings allowance.  They are not free of inheritance tax. 
Thank you.