Hello Shaun,
I’m not saying your car is sitting level, but heights should be measured from the wheel centre to the underside of the wing, (not to the ground) with the car sitting on level ground.
Possibilities;- New spring has now settled, and even fitting a new matching one on the other side, may result in the car not sitting level in the first place.
Quality of spring? ‘Third part spring’ . New term for me, but tempted to think this is another phrase for ‘aftermarket’, and could equal not good quality.
The garage must inspect and repair the car, since it assumed that it sat level when the new spring was first fitted - and now 4 months later, it doesn’t. New spring broken again?
Kind regards,
Gareth.