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I'd be looking at getting the rocker cover off to check the cam for scoring and wear, I'd get a compression check and see what is going on. I'd aslo want the sump off to check the big ends for scoring. Also to clean out the sump as the oil level drops the oil gets cooked and becomes thick and sticky.
On a 4 year old motor, I'd want this done. The garage has insurance to cover this.
EDIT - I'd also want a cyclinder leakage/leak down test.
Been planning on asking them to clean the engine bay as it is now covered with oil due to their incompetence and some form of refund on the service/compensation for wasting a load of my time and risking the death of the car etc