Now the damper's out, you'll need to remove the parts attached to it, as you'll need these to re-fit the damper. You'll need a 17mm spanner to hold the hexagon shaped extension bar still, whilst you undo the securing nut with a 13mm spanner. Unscrew the hexagon bar, but you shouldn't need the nut, as your new damper should come with one. Check though, before you sling it over the hedge!
Old damper on the left and the parts to be re-used and the new damper on the right:
Now reassemble the damper assembly, with the new damper, slide it back inside the coupling and resecure it on the rear bolt.
Fit the coupling back on to the trailer, use a couple of new M12 locking nuts if you can, as they'll be old and you don't want them coming loose... Then re-attach the brake rod.
Now for the bit you have to pay attention to! You need to line the damper eye up with the couping bolt hole. As a rule damper's will extend past the coupling eye bolt by around half a hole:
So, using the trusty 'spike' that all men have in their toolbox (invariably a worn down / ground to a point old screwdriver!), you need to line the damper eye up from the opposite side that you're putting the bolt through:
Now stick the coupling head back on and hold in place with the front retaining bolt:
Now, as you push the rear bolt in you may need to lever/wiggle the 'spike' to keep the damper lined up. As you push the bolt in carefully remove your spike / screwdriver and the bolt will now be through the coupling head, the drawbar, the damper eye bolt and out the other side. In the next pic you can see the bolt part way in and the 'spike' at an angle, due to the pressure from the damper wanting to expand. Just lever against this force and the damper eye will stay lined up.
Once this bolt is through then it's time to tighten them all up and you're done! Congratulations, you've just changed your damper. If that doesn't deserve a brew and a biscuit I don't know what does.
The eagle eyed amongst you will have noticed there's a new breakaway cable on there too. A good tip for you is to use a D-shackle between the breakaway cable and where it locates on the bottom of the handbrake. If you've ever tried prying those sprung rings apart and winding them on to the bottom of the handbrake it usually ends with a screwdriver stab to the palm, a blood blister or, at the very least, copious amounts of cursing.