Car uneven at rear

My 20v NA, 1999 with 93k miles, rides some 20mm higher at the rear passenger side. My garage, who is very knowledgeable and coupe experienced, cannot find a specific cause. Clearly several old age matters may have an accumulative result. Any bright ideas? Thanks Paul.

Do the tyres match, broken spring, damaged/bent trailing arm, gold ingots hidden in boot.......................

Very large previous owner, mostly driven without a passenger?

Have you tried letting the handbrake off?

My handbrake has gone dodgy on one side (given they dont work anyway you could hardly tell tbf...) but if it was on even a slight incline the on the handbrake it'd lift on the non working side

Thanks for that.
Handbrake is fine.
Car has been on lift at garage and thoroughly checked.
Nothing is jumping out as being at fault.

That's what we were wondering.