P3 rear spring shackle adjustment

[byb]

I am trying to level up the rear suspension after new springs fitted and the car has a 1” list. I have read the w/s manual item on doing this by slackening off the shackle bolts and winding in or out the threaded adjustment bolt. This makes no measurable difference to the height measured from a level floor to the underside. I have checked that both tyres have equal pressure. If anyone has experience of setting up the rear suspension and they can impart their wisdom I should be very grateful.