Yes its just a normal Al-Ko axle, you can't do anything with the suspension, you have to replace complete axle. If its only 3/4inch now then thats near enough, plenty of caravans are at least that much out so I wouldn't worry about it.
My twin axle trailer does this....wheels off in winter, first time out it bounces around and rides high but after a couple of loads in it it soon settles down