You need to renew one or preferably both, gas struts.
See here for info-
http://www.strutsdirect.co.uk/components/how_to_replace_car_boot_gas_strut.php
The method may not be exactly the same for yours, but you get the idea.
There other suppliers, check also local motor factors They are often cheaper than internet by the time you pay for postage, and you dont have to stay at home waiting for the delivery!!
vaux, November 2009