You'll need to check, as presumably you will have (or should have anyway!) - unless it was an approved profit sharing scheme - you really need to find out what type of structure/arrangement you were given the shares under.
The reason being that whatever value you acquired them at, and were charged income tax on, will be treated as the cost for CGT purposes - you'll be liable for CGT on any increase in value since. I'm pretty sure we had a similar thread recently...