I was asking about tax because sometimes companies can allow employees to purchase shares at less than market value - either via a share option or by means of an employee share purchase plan where you can get e.g. 15% discount on then-current market price. The fact that you use the word "matured" when you talk of the shares implies to me that you were granted shares on date X and that you only get to buy/get them on date Y and that the share price was either set on date X or you get them for free. The question is whether, on the date they vest, the open market value of the shares greater or less than the price you paid. If you didn't pay anything and you were just granted the shares after a certain period, then you should talk to your employer's HR department and look at the share plan to determine if you have to pay income tax. I don't know myself how those particular schemes work with regard to tax but, for example, if you paid 15% less than then-current market value for the shares when they vested, you would have to pay income tax on that 15% differential within 30-days of the shares vesting.
With regard to CGT, yes, you will have to pay CGT on the difference between market value on the date the shares vested and the market value on the date you sell. The market value on the date the shares vested will be 100% of the market value, assuming you've paid the necessary income tax, if any (i.e. it's not based on the 85% of market value that you may have paid if you use my example above). The CGT allowance applies (1,270 per annum) and you would pay CGT on any amount gained over that. If the shares go through the roof and are worth more than 1,270 more than the market value when they vested, then you can split out sales between tax years to take advantage of an additional CGT allowance. You can also offset realised losses against the gain, which you have to do before you take that CGT allowance.
Hope this helps
Sprite