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Has any work been carried out on the rear suspension in the recent past ?
If so, I'd check that the springs were correctly pre-loaded to the Factory's recommendation before the relevant fasteners were tightened.
When I got my current S1 coupe it was sitting very high at the back so I slackened everything off, put the rear up on axle stands & added 'ballast' to the boot (I used new bags of gravel) until there was a gap of 80mm between the spring plate & the bump stop rubber - in my case it took 5 x 25kg bags - then tightened all the fasteners to the recommended torque.
The car then looked (&, more importantly, drove) as it should !
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