To get the right damper you need to look on the A frame, under the cover and you shoud find(for an Al-Ko Chassis) a number starting with 811***, then go to
http://www.western-towing.co.uk/acatalog/Al_ko_Kober.html and match your 811*** number with the correct damper.
Assuming you have an Al-Ko chassis, remove the hitch head by the two bolts going through the side of the hitch head, and one bolt vertically at the rear(nearest the van).
The hitch damper should slide out towards the van, make sure you catch the spacer plate which is at the rear as it drops out.
If there is not enough room for th edamper to slide out, undo the 4 bolts holding the hitch assembly to the chassis and tilt the hitch to allow room.
Make sure the new damper has the locating eye at 90 degrees so as to line up with the fixing bolt at the front, and the rear locating eye vertical to line up with the rear bolt.
Slide it in replacing the spacer and fix with the rear bolt, retighten the 4 bolts of the hitch if you have undone them to get the damper out.
Fit the hitch head with the front bolt ONLY, then compress the hitch as far as it will go and pull it out quickly, and with a suitable screwdriver or rod, catch the front fixing eye as it extends into the hitch, refit the front bolt and torque all up to their recommended torque figure. (you may have to practice catching the eye several times, and repeat the compression of the hitch until you catch the eye.)