2004 Renault Clio Smoke blowing from exhaust

I recently had the timing belt replaced on my 2004 clio dci 1.5 and it also had a full service. When driving it home from garage smoke started blowing from exhaust when I slowed down at junctions and then acellerating off. I've since returned the car to the garage where they said they'd changed the oil to a different gradient but it had made no difference. Now they tell me that it could be the turbo letting in oil but the car was running normally before I had the work done so why would it be that the turbo is to blame