G'day Paul,
I get a box of flat washers and insert between the coils until the spring is long enough to fit.
Bend the spring to insert and alternate from side to side.
Trev.
I replaced the spring on my Shooting star today and I developed this idea slightly. I used oval nails. I pushed four in between the coils of the spring (two on each side to keep it straightish) I then used a pair of pliers to rotate each nail like a cam, forcing the coils further apart. This achieved me about 5mm and the spring just popped straight on. I then used the pliers to pull the nails out.