"Leaking" or "misting"?
I had a "misting of oil on a shock absorber" on the first MOT after owning my car, and it has never been mentioned again. Either the oil has run out, or it wasn't a misting of oil...
How long had you owned it when you first reported the fault?
If within 30 days, in theory, you have the right to reject as its faulty. But at 6 weeks now... you appear to be outside of that window.
If more than 30 days and up to 6 months, you have to give the dealer one chance to repair/replace/refund (their discretion), the onus is on them to prove the fault was not present at time of purchase.
Depending upon the year of car, you may find that the car has had less services than you would expect. These are solid cars (again, depending on age) if well looked after, and if you do the appropriate miles for a diesel to do its regens you should have no issues once these are all solved.