I think you can do this via a stockbroker, bank or building society, but they may charge commission. You will need your shares certificate. Depending on the profit that you make, there may also be capital gains tax implications.
If you sell them on barclays sharedeal, they will charge you a lesser dealing fee by selling them online than if you were to sell them over the phone or buy post
There was a radio programme on this yesterday, http://news.bbc.co.uk/1/hi/programmes/moneybox /6378635.stm
if you have broadband you can listen again, or get the transcript when it comes up. I think they gave a web address for cheap selling.