Have you tried changing the batteries on the remote? And a factory reset of the YouView box (power off at the mains, switch on immediately pressing and holding plus and minus keys on the front of the unit)
Failing that, you can but a replacement at http://accessories.talktalk.co.uk/product_details.aspx?idProduct=34
Failing that, you can but a replacement at http://accessories.talktalk.co.uk/product_details.aspx?idProduct=34