There is an easy way and a hard way.... The above being the hard way
It all depends if you want to spend 2hrs per side or 45 minutes per side
Bear in mind... The more parts you have to remove, the more chance you have of problem nuts and bolts being siezed etc.... Keep it simple!
The easy way:
Buy new upper wishbone nut and bolt sets (part numbers):15505821 x 2 and 82427709 x 2
it pays dividends later on and in the future £21.70 inc vat
from the dealers for the pair
Loosen the front wheel bolts, hand brake on
Jack up the front end and place axle stands under the front cross member
Remove the the front road wheels.
Remove the front wheel arch covers.
Remove the two black boxes in the rear of the upper wheel arches (one is a carbon filter, the other is an alarm box) just to make more room
Remove the ball joint securing nut for the upper arm and either use a ball joint splitter or two large hammers together and bang the joint either side at the same time (watch it just pop)
Pull the wishbone downwards now for better access
Loosen the long nut and bolt that go through the bushes but dont remove the nut totally off the thread.
Knock the bolt through towards the coil spring as far as it will go....
Now this is where most people get confused and insist on doing it the hard way as they can't see how to get the bolt out past the spring!
You should have the bolt out about 2" proud by now... Get hold of either an angle grinder with a cutting disc or a junior hacksaw.
Now cut the bolt as near to the wishbone as you can.
Clean up any swarf with a file on the cut as you will now be pulling the bolt back through from the nut end... It will come, trust me!
Simply let the wishbone drop away from the top mount section and fit the new wishbone in reverse
This time though make sure you fit the bolt in the opposite way so the threads go in towards the spring.
Now next time you have to replace them it will be even easier as no need to cut the bolt
Slot the ball joint in the hole of the hub leg and tighten up.
Refit the black boxes and the wheel arch liners.
refit road wheels, jack it up to remove the axle stands and tighten up the wheel bolts....
Job done... Have a cuppa and be proud of the fact that you haven't had to remove/ disturb half the suspension to do the job properly and your geometry is intact
Like I said... an easy way and a hard way