my 18mm wishbone nut is actually loose ive just check but i cant get a spanner in to stop the top nut from spinning...any tips?

mine did this.. you want a "deep" spanner to squeeze between the sub-frame and chassis rail onto the nut. If you approach it from the back of the car you should be able to just get a normal spanner in, then wedge it on the chassis rail while you tighten the bolt.