The nut you can see is the nut on the dowel bolt that goes through the spring leaves and holds the spring leaves together. There is absolutely no need for it to be undone.
As pointed out by Les the bottom plate which mounts the damper has a large hole at its bottom centre and sits over and around the spring leaf dowel bolt/nut.
Your new springs will have the dowel bolt/nut holding the leaves together.
Below the actual spring you have will be:-
The rubber spring pad.
The metal rubber spring pad seat/axle locator into which the rubber spring pad seat sits.
The bottom plate with damper.
Above the spring you will have:-
The rubber spring pad.
The metal rubber spring pad seat/axle locator into which the rubber spring pad seat sits
The axle mounting bracket.
The head of the dowel bolt sits inside the rubber spring pad which in turn sits inside the axle mounting bracket.
All these parts are held together by the 'U' bolts.
See link below for exploded parts diagram:-
http://www.moss-europe.co.uk/shop-by-mo ... minor.html
It should be noted that the front half of the spring will have more spring clamps than the rear half of the spring. The larger number of clamps indicate the front of the spring and should be fitted accordingly.
There is no part number quoted in the link for the dowel bolt/nut as they come supplied as part of the complete spring.