Best way to procede
put your jack under the lower arm, raise the vehicle and support the vehicle under the chassis rail. keep the jack where it is for the time being.
Remove the wheel, undo the 3 bolts in the bottom ball joint to arm junction, if the arm is moved up or down with the jack, you'll be able to just pull the ball joint straight out of the arm without risk to yourself.
undo the nut on the rear bolt in the lower arm and the nut on the front bolt, you should push the arm down enough to get the old spring out, if it's not remove the rear bolt and the arm will drop with loads of clearence.
Push the new spring into position in the subframe and get someone to make sure it stays put while you use the jack to move the lower arm back into position, you might need to just pry the arm back in place if you've had the rear bolt out. Once the arm is square there, continue to jack it up and fit the ball joint back into the arm.
Should take you less than 1.5 hours and there is no need for a spring clamp.
Hope it all goes well for you, i'll not be around to help though for a week as i'm off down to Daventry to have a new chip inserted in my noggin
