I contacted MB regarding handing back our vehicle 2 weeks early as our new vehicle becomes available within the 3 month time allocated to pre order but it is considered the 3 year lease has not been fulfilled.
the conversation indicated that it would cost £50 to have the lease curtailed by 2 weeks and the good condition bonus would be lost due to the lease being reduced.
with the current constraints on new vehicles and the ability to order upto 3 months early i would have thought some leeway to facilitate a very short period either way to allow for eventualities such as this.